landform regolith mapping in the west african context abstract in this paper we present an integrated landform regolith mapping protocol chart for west africa developed using a case from central burkina faso the chart accounts for i the specificities of the ubiquitous duricrust capped remnants of the regional paleolandsurfaces particularly the residual or detrital character of the fe duricrusts and ii the diversity of the remaining non duricrusted portions of the landscape with an emphasis on the pitfalls encountered in determining the in situ or transported character of the regolith the paper also provides field identification criteria and illustrations of the typical west african landform regolith associations the landform regolith mapping chart is integrated into a prospectivity chart which is used to define exploration guides i defining sampling strategies ii interpreting potential geochemical anomalies and iii targeting lateral extensions of known bedrock mineralizations the mapping protocol further allows for the definition of a class of landform regolith association propitious to supergene mineral concentrations the mapping protocol is shown to be transposable to wetter contexts and more generally to other tropical shield provinces

relief inversion in the geomorphological evolution of sub saharan west africa abstract the geomorphology of much of sub saharan west africa is dominated by the presence of plateaux and plains with ferruginous and locally aluminous bauxitic duricrusts the plateaux occur at different elevations and have been correlated as two or more palaeosurfaces across much of the region the duricrusts have generally been considered to be residual formed by conformable erosion and chemical wasting of immediately underlying bedrock this concept has been central to interpretations as diverse as the formation and evolution of the landscape and the development of geochemical exploration models recent regolith landform mapping field observations and experience from mineral exploration in southern mali and burkina faso however demonstrate that the duricrusts are mainly ferricretes i e fe oxide cemented sediments these observations require a re interpretation of the geomorphological evolution of the region during the cenozoic the landscape has evolved by several cycles of weathering and erosion deposition triggered by climatic tectonic or other environmental changes it is proposed that an initial bauxitic lateritic regolith was partly eroded following uplift and or a change to a more arid climate and that the detritus rather than being removed was deposited on slopes and valleys during a subsequent humid period of lateritic weathering fe oxide cementation of this detritus formed ferricrete dehydration and hardening of the ferricrete after further uplift or aridity increased its resistance to erosion resulting in relief inversion with the detritus in turn being deposited downslope this too has been weathered and cemented to form a younger ferricrete the occurrence of ferricrete landforms in adjacent countries noted by field observation and inferred from satellite imagery demonstrates that relief inversion is a very widespread and important phenomenon in southern mali burkina faso and adjacent countries in semi arid west africa

a geophysically constrained litho structural analysis of the eburnean greenstone belts and associated granitoid domains burkina faso west africa abstract interpretation of airborne geophysical data integrated with field structural and lithological observation was successfully employed in the creation of the litho structural framework in a poorly exposed paleoproterozoic granite greenstone terrain of the west african craton the geophysical data portray with sufficient detail all of the lithological units and structural features present the results suggest that the granitoid domains are formed by numerous small to medium sized plutons the existence of several generations of magmatic episodes has a significant impact on the development of a regional tectonic model the magnetic data provided a better definition of the actual pluton shapes and several highly magnetic late orogenic plutons were reliably identified relic f1 fold hinges were recognized in the hounde and boromo greenstone belts the d1 penetrative structures are generally affected by the d2 transcurrent shear zones which often form as well the granite greenstone belt boundaries previously unreported s2 shear zones including the bossie shear zone transecting the sideradougou granitoid domain and abundant small scale shear zones affecting all of the lithologies represent prospective areas for gold exploration airborne gamma ray spectrometry data aided in the mapping process in areas with less regolith cover including erosional ridges and parts of the banfora greenstone belt the magnetic data revealed at least three generations of doleritic dykes crosscutting the western burkina faso

introduction to the special issue challenges for mineral exploration in the 21st century targeting mineral deposits under cover abstract the global demand for mineral resources has dramatically increased in the last 50 years the challenge to keep pace with this demand is the driving force for innovation and also drives mineral exploration in regions where mineral deposits are under cover and therefore not exposed or near surface for which novel techniques are required to locate these mineral deposits hidden beneath this often hundreds of meters thick cover material generalizing regions under cover can be globally divided into three end member types 1 regions with intensely weathered cover in tropical or dry desert and semi arid climatic domains based on koppen geiger s climate classification 2 regions covered by sedimentary sequences that can span from the proterozoic to the cenozoic and glacial sediments and 3 regions under water seawater freshwater or ice this special issue aims to contribute to the debate in the field of mineral exploration in regions that are dominated by and present the complexity of cover by presenting different case studies and technology approaches to detect the distal footprint of mineral systems and deposits at depth

shredding of environmental signals by sediment transport 1 landscapes respond to climate tectonic motions and sea level but this response is mediated by sediment transport understanding transmission of environmental signals is crucial for predicting landscape response to climate change and interpreting paleo climate and tectonics from stratigraphy here we propose that sediment transport can act as a nonlinear filter that completely destroys shreds environmental signals this results from ubiquitous thresholds in sediment transport systems e g landsliding bed load transport and river avulsion this morphodynamic turbulence is analogous to turbulence in fluid flows where energy injected at one frequency is smeared across a range of scales we show with a numerical model that external signals are shredded when their time and amplitude scales fall within the ranges of morphodynamic turbulence as signal frequency increases signal preservation becomes the exception rather than the rule suggesting a critical re examination of purported sedimentary signals of external forcing citation jerolmack d j and c paola 2010 shredding of environmental signals by sediment transport geophys res lett 37 l19401 doi 10 1029 2010gl044638

automated regolith landform mapping using airborne geophysics and remote sensing data burkina faso west africa abstract we have studied the regolith landform distribution in the area of gaoua western burkina faso using an integration of geophysical and remote sensing data concentration maps of k th u as well as their ratios were computed from airborne gamma ray spectrometry data to assess the geochemical composition of the regolith the mineralogy of the surfaces was mapped via the analysis of multispectral aster and landsat scenes pauli decomposition data retrieved from polarimetric alos palsar and radarsat 2 images were included to characterize the surface properties of the regolith material morphometric variables such as slope curvature and relative relief were derived from the srtm digital elevation model to quantify the topographic parameters of the different regolith landforms an artificial neural network implementation advangeo was then employed to extract four basic regolith landform units from the satellite and airborne data relic ferruginous duricrusts rich in hematite and goethite belonging to the high glacis erosional surfaces represented by rock outcrops and suboutcrops alluvial sediments and soft pediment materials of the middle and low glacis were mapped successfully in the region the results were compared with the existing geomorphological maps an independent visual classification and field observations we found that the distribution and shape of the iron rich duricrusts are more accurate than portrayed in the current maps the best results with an overall accuracy of 94 21 and a kappa value of 0 92 were obtained for a dataset consisting of gamma ray spectrometry data combined with derivatives of the srtm digital elevation model augmented by landsat and polarimetric radar data the approach demonstrates for the first time the potential of machine learning in regolith landform mapping the proposed combined analysis of airborne geophysics and remote sensing data can be adopted easily in other regions with similar long term lateritic weathering histories worldwide

early oligocene stratigraphic turnover on the west africa continental margin a signature of the tertiary greenhouse to icehouse transition the post rift stratigraphy on the west african margin is characterized by aggradation of a carbonate ramp during late cretaceous to eocene epochs and progradation of a terrigenous wedge from oligocene to the present such first order structure has been attributed in the past to geodynamic forcing however comparison of the stratigraphic record of the margin with eustasy 18o and 87sr 86sr curves shows a close temporal relationship with the tertiary climate cooling an increase of continental weathering and a long term lowering of sea level we suggest that the transition from low amplitude high frequency sea level changes during the greenhouse period to high amplitude high frequency sea level changes during the icehouse period may account for i the switch from an aggrading carbonate ramp to a prograding clastic wedge and ii the enhanced continental weathering and increased terrigenous influx to the margin

geomagnetic variation anomalies and deflection of telluric currents summary we show that most of the abnormal variations of the transient magnetic field have deflection and canalization of telluric currents as main sources in order to do so we develop a formalism which is valid in a limited frequency range which satisfies the following hypotheses a the thickness of the laterally inhomogeneous upper layer is small with respect to the penetration depth of the transient field b self induction of the heterogeneities of conductivity can be neglected theoretical results and observations performed at mid latitudes are in good agreement the main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations the study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced

contribution of small glaciers to global sea level observed long term changes in glacier volume and hydrometeorological mass balance models yield data on the transfer of water from glaciers excluding those in greenland and antarctica to the oceans the average observed volume change for the period 1900 to 1961 is scaled to a global average by use of the seasonal amplitude of the mass balance these data are used to calibrate the models to estimate the changing contribution of glaciers to sea level for the period 1884 to 1975 although the error band is large these glaciers appear to account for a third to half of observed rise in sea level approximately that fraction not explained by thermal expansion of the ocean

the rotational stability of an ice age earth summary predictions of glaciation induced changes in the earth s rotation vector exhibit sensitivities to earth structure that are unique within the suite of long wavelength observables associated with glacial isostatic adjustment henceforth gia and despite nearly a quarter of a century of research these sensitivities remain enigmatic previous predictions of present day true polar wander tpw speed driven by gia have indicated for example a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment phase or chemical of the density discontinuity at 670 km depth nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere however his results have introduced an intriguing paradox in particular predictions generated using a model with an extremely high viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness mitrovica as cited by nakada has suggested that the paradox originates from an inaccuracy in the traditional rotation theory e g wu peltier associated with the treatment of the background equilibrium rotating form upon which any load and rotation induced perturbations are superimposed we revisit these issues using a new treatment of the linearized euler equations governing load induced rotation perturbations on viscoelastic earth models we demonstrate that our revised theory in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity resolves the apparent paradox calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors indeed previously noted sensitivities of tpw speed predictions to the thickness and rheology elastic versus viscous of the lithosphere largely disappear in the application of the new theory significant errors are also incurred by neglecting the stabilizing influence of the earth s excess ellipticity finally we demonstrate that the contribution from rotational feedback on predictions of present day rates of change of the geoid sea surface and crustal velocities are overestimated by the traditional rotation theory and this has implications for analyses of ongoing satellite e g grace missions and geodetic gps surveys

constraints on mantle anelasticity from geodetic observations and implications for the j2 anomaly summary we use geodetic observations of the earth to constrain anelasticity in the earth s mantle at periods between 12 hr and 18 6 yr the observations include satellite laser ranging slr measurements of 12 hr and 18 6 yr tides in the j 2 component of the gravity field spacebased observations of tidal variations in the earth s rotation rate and optical and space based measurements of the chandler wobble period and damping these geophysical signals are mostly sensitive to the lower mantle the results suggest the dissipative process could consist of a single absorption band that extends across seismic periods out at least as far as 20 yr the results also require values of the anelastic parameter q that are smaller than those required by seismic observations we interpret this as evidence that q in the lower mantle is frequency dependent the frequency dependence suggested by the geodetic observations is reasonably consistent with laboratory measurements though those measurements have only been done on rocks at upper mantle conditions after fitting and removing the 18 6 yr tide from the slr j 2 results we find that the 1998 2002 anomaly present in the original j 2 observations is no longer a singular anomaly in the j 2 residuals but becomes one of a series of maxima in a quasi decadal oscillation

azimuthal flows in the earth s core and changes in length of day at millennial timescales summary variations in the length of day lod reconstructed from ancient records of eclipses contain an oscillating component with a periodicity of roughly 1500 yr a part of the time dependent variations observed in archaeomagnetic field models consist of episodic eastward and westward motions with a similar periodicity using standard inversion techniques we obtain time dependent azimuthal flows at the surface of the core that can explain parts of the archaeomagnetic secular variation and with amplitude and characteristic timescale of variation consistent to explain the lod signal on this basis we argue that exchanges of angular momentum between core flows and the mantle are responsible for the observed millennial oscillations in the earth s rotation rate however we do not obtain a detailed match between the observed changes in lod and the prediction based on simple flow profiles inside the core we argue that this is due to the presence of time dependent azimuthal flows that have an important shear in the direction of the rotation axis such flows are interpreted as oscillations in thermal and magnetic winds

long term evolution of the west african transform margin estimates of denudation from benin using apatite thermochronology the benin continental margin was formed during the breakup of gondwana through oblique rifting along transform faults the evolution of topography following breakup directly affects the evolution of sedimentary basins which has major implications for hydrocarbon exploration in the region quantitative constraints on erosion across benin are limited to the cenozoic based on analysis of dissected lateritic palaeolandscapes to resolve the mesozoic erosion history we have obtained apatite fission track and single grain u th sm he data from 18 samples collected across a 600 km long transect through benin we invert these data including available geological and geomorphological constraints to obtain time temperature paths which are used to estimate magnitudes of denudation over the last 200 myr our study suggests that denudation was focused over a c 300 km long seaward sloping limb of the marginal upwarp and at the southern margin of the interior iullemmeden basin from 140 to 100 ma with lower magnitudes of denudation characterizing the continental interior and post cretaceous evolution of the margin models are consistent with modest burial c 1 km of the iullemmeden basin between 120 and 85 ma and of the continental margin between 85 and 45 ma by the eocene the first order relief of benin had developed with regional erosion rates supplementary information full details of the analytical data and modelling results including the various constraints and further data on denudation and burial magnitudes and rates are available at https doi org 10 6084 m9 figshare c 4220804

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

40ar 39ar dating of west african lateritic cryptomelanes abstract dating weathering processes is an innovative approach to understanding climate dependant modifications of the earth s surface potassium bearing manganese oxides are a ubiquitous feature of weathering mantles developed from mn bearing parent rocks recent studies have demonstrated the possibility of dating k rich manganese oxides from the hollandite group such as cryptomelane k 1 2 mn 8 o 16 nh 2 o using 40 ar 39 ar argon ion laser probe method this work applies that approach to cryptomelanes formed in the lateritic manganese ore deposit of tambao northern burkina faso a present unexploited deposit among the cryptomelane richest in the world two samples of massive mn rich concretions 10mm and 4mm in thickness were collected from the massive 80 m high mn rich hill at tambao a detailed petrographical study shows that these cortical formations are composed of successive na rich cryptomelane layers with na na k ba up to 20 seven and five subsamples from these two concretions were studied with a 40 ar 39 ar laser probe the age spectra indicate ages that are coherent with the general organization of the concretions showing a regular decrease from cores towards rims within the two concretions ages range from 49 8 0 4 ma to 48 1 0 1 ma and from 47 5 0 3 ma to 44 5 0 4 ma respectively the youngest ages being obtained for the most external layers the integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm ma these ages clusters show that weathering at tambao was functioning during the early eocene and thus suggest that there is a relationship between the formation of the bauxites of the african surface and the mn fe laterites

mountain ranges climate and weathering do orogens strengthen or weaken the silicate weathering carbon sink abstract the role of mountains in the geological evolution of the carbon cycle has been intensively debated for the last decades mountains are thought to increase the local physical erosion which in turns promotes silicate weathering organic carbon transport and burial and release of sulfuric acid by dissolution of sulfides in this contribution we explore the impact of mountain ranges on silicate weathering mountains modify the global pattern of atmospheric circulation as well as the local erosion conditions using an ipcc class climate model we first estimate the climatic impact of mountains by comparing the present day climate with the climate when all the continents are assumed to be flat we then use these climate output to calculate weathering changes when mountains are present or absent using standard expression for physical erosion and a 1d vertical model for rock weathering we found that large scale climate changes and enhanced rock supply by erosion due to mountain uplift have opposite effect with similar orders of magnitude a thorough testing of the weathering model parameters by data model comparison shows that best fit parameterizations lead to a decrease of weathering rate in the absence of mountain by about 20 however we demonstrate that solutions predicting an increase in weathering in the absence of mountain cannot be excluded a clear discrimination between the solutions predicting an increase or a decrease in global weathering is pending on the improvement of the existing global databases for silicate weathering nevertheless imposing a constant and homogeneous erosion rate for models without relief we found that weathering decrease becomes unequivocal for very low erosion rates below 10 t km2 yr we conclude that further monitoring of continental silicate weathering should be performed with a spatial distribution allowing to discriminate between the various continental landscapes mountains plains

simultaneous inversion for the earth s mantle viscosity and ice mass imbalance in antarctica and greenland redistribution of mass in the earth due to pleistocene deglaciation and to present day glacial melting induces secular changes in the earth s gravitational field the earth is affected today by the former mechanism because of the viscous memory of the mantle and by the latter because of ongoing surface mass redistribution and related elastic response a self consistent procedure allows us to invert simultaneously for the lower and upper mantle viscosity and for the present day mass imbalance in antarctica and greenland using the observed time variations of the long wavelength gravity field from satellite laser ranging slr analyses the procedure is based on our normal mode relaxation theory for the forward modeling and a newly developed inversion scheme based on the levenberg marquardt method we obtain a large viscosity increase across the 670 km depth transition zone separating the upper and the lower mantle with the lower mantle viscosity varying over the range 5 1021 to 1022 pa s and the less resolved upper mantle viscosity of the order of 1020 pa s when antarctica is the only present day source its rate of melting is 240 gt yr 1 corresponding to a sea level rise of 0 7 mm yr 1 when greenland is added as a source of ice loss the rates of melting are 280 gt yr 1 for antarctica and 60 gt yr 1 for greenland corresponding to sea level rises of 0 8 and 0 2 mm yr 1 slr data indicate that ice melting in the polar regions of the earth is ongoing

weathering and the global carbon cycle geomorphological perspectives abstract the links between the weathering of silicate minerals the global carbon cycle and climate change are complex and involve a whole range of often inter related tectonic climatic geomorphological and biological factors silicate weathering rates are known to have varied through geological time and increasingly sophisticated models now link such variation to the long term carbon cycle and climate change in order to improve the modelling further one key research need is for more and better datasets on weathering and the factors that control it as a contribution to this we review the nature of the weathering carbon cycle system the sources of evidence used to understand it and the challenges that remain in quantifying the feedbacks between weathering and the global carbon cycle we propose that increased understanding of geomorphological diversity of landscapes and processes and especially the dynamic relationships between weathering and erosion will aid clearer portrayal of the links between weathering and the long term carbon cycle

global geodetic signatures of the antarctic ice sheet four scenarios of present day antarctic ice sheet mass change are developed from comprehensive reviews of the available glaciological and oceanographic evidence the gridded scenarios predict widely varying contributions to secular sea level change ranging from 1 1 to 0 45 mm yr and predict polar motion and time varying low degree gravitational coefficients that differ significantly from earlier estimates a reasonably linear relationship between the rate of sea level change from antarctica a and the predicted antarctic is found for the four scenarios this linearity permits a series of forward models to be constructed that incorporate the effects of ice mass changes in antarctica greenland and distributed smaller glaciers as well as postglacial rebound assuming the ice 3g deglaciation history with the goal of obtaining optimum reconciliation between observed constraints on and sea level rise numerous viable combinations of lower mantle viscosity and hydrologie sources are found that satisfy observed in the range of 1 to 2 2 5 mm yr and observed for degrees 2 3 and 4 in contrast rates of global sea level rise above 2 5 mm yr are inconsistent with available observations the successful composite models feature a pair of lower mantle viscosity solutions arising from the sensitivity of to glacial rebound the paired values are well separated at mm yr but move closer together as is increased and in fact merge around 2 2 5 mm yr revealing an intimate relation between and preferred lower mantle viscosity this general pattern is quite robust and persists for different solutions for variations in source assumptions and for different styles of lower mantle viscosity stratification tighter constraints for l 2 may allow some viscosity stratification schemes and source assumptions to be excluded in the future for a given total observed the sea level rise from antarctica a is tightly constrained and ranges from 0 to 1 mm yr corresponding to an ablating ice sheet as estimates of are raised from 1 to 2 5 mm yr however when the degree 3 zonal harmonic constraint is removed the solutions show little sensitivity to antarctic mass balance emphasizing the need for a well determined odd degree secular zonal harmonic for determining polar ice mass balance

the contribution of mountains to global denudation the hypothesis that mountains infl uence global climate through links among rock uplift physical and chemical denudation and the carbon cycle remains vigorously debated we address the contribution of mountains to global denudation with an empirical model that predicts that 50 of the total denudation and 40 of the chemical denudation occur on the steepest 10 of earth s terrestrial surface these fi ndings contrast with those from a recent study that suggested global scale denudation occurs primarily on gently sloping terrain but did not account for the infl uence of digital elevation model resolution on modeled denudation rates comparison of calculated denudation rates against the sum of measured sediment and solute yields from 265 watersheds indicates a positive correlation r 2 0 44 with order ofmagnitude variability refl ecting among other things the effects of dams and agriculture in addition ratios of measured river yield to modeled denudation rate decline as catchment area increases due to progressively greater sediment storage with increasing drainage area our results support the conclusion that the small mountainous fraction of earth s surface dominates global denudation and the fl ux of sediment and solutes to oceans

physical properties of the variations of the electric field of the earth preceding earthquakes ii determination of epicenter and magnitude abstract as reported in the preceding paper a transient change of the electric field of the earth seismic electric signal hereafter called ses appears many hours before an earthquake eq by measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction measurements in two directions e w and n s give the relative signal intensity jrel at the station under consideration by measuring jrel at a number of stations and considering that it attenuates according to a 1 r law the epicenter can be determined with an accuracy usually around 100 km once the epicenter has been determined the product jrel r can be evaluated so that the magnitude m can be estimated by resorting to an empirical log jrel r versus m plot the uncertainty of m is around 0 5 units following sobolev 1975 and for the statistics to be beyond any doubt predictions were officially documented before the eq occurrence for 23 earthquakes with a magnitude equal or greater than ms 5 0 two events were missed the present method is compared to other electrical methods used in china japan and soviet union a number of problems concerning the origin of the effect its directivity and the attenuation with distance remain open for further studies

west african lateritic pediments landform regolith evolution processes and mineral exploration pitfalls this paper is a contribution to the understanding of surface dynamics of tropical shields over geological timescales emphasis is put on the fundamental and applied implications of regolith production and dispersion processes through the formation dissection and preservation of landforms it is based on the case study of sub saharan west africa which recorded neogene 24 3 ma stepwise dissection of its topography through the emplacement of three lateritic pediment systems which still occupy most of its surface pediments are erosional transportation slopes having been weathered and duricrusted pediment regolith associations therefore depend on the parent rock transport dynamics and preservation of the material having transited on their surface as well as on the intensity of their weathering duricrusting iron oxy hydroxide cemented clastic sediments detrital ferricretes and unconsolidated clastic sediments are the dominant outcropping material and as such represent a challenge for mineral exploration that relies on surface geochemical sampling to detect metal concentration in the bedrock landform regolith mapping beyond the scale of modern interfluves combined with paleolandscape reconstitution are relevant to provide exploration guides for i interpreting geochemical anomalies on pediments ii tracing their potential source when they have been transported on pediments and iii targeting suspected ore bodies concealed beneath pediment s past and present latitudinal climatic zonation of pedimentation and weathering patterns suggests a gradation of pedimentation process across the intertropical zone and explains why pediments may have been overlooked in equatorial environments with implications for mineral exploration successive pediment systems adapted to uneven knickzone bearing river networks producing a spatially consistent and reduced 80 m stepping pattern of pediments independent from elevation or distance to base level pediments pediplains are therefore not proxies of uplift and their preservation over geological timescales typifies regions submitted to 10 m my erosion rates the identification and study of lateritic pediments bear important implications on shield sediment routing systems and a better access to the bedrock and its resources which may still be underestimated in the tropics

tectonoelectric zonation in the hellenic arc abstract the electrical apparent resistivities were measured by means of the magnetotelluric effect at twenty sites in greece and in two directions e w and n s in most of the sites ew differs considerably from ns these results allow the following three zones to be distinguished 1 1 an external zone along western greece with ew ns 2 2 an intermediate zone along the main mountain chain in continental greece where ns ew and 3 3 an internal zone in the northern aegean where ew ns the above resistivity zones are compared to data from geotectonic neotectonic seismotectonic and in situ stress measurements which all show a similar geometry of zonation almost parallel to the active hellenic arc

modes tempo and spatial variability of cenozoic cratonic denudation the west african example 1 quantifying long term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of co 2 and sediments in the context of long term cenozoic climate change it is also a key to understanding long term landform evolution processes operating over most of the continental surface and their control onto the sediment routing system we study the surface evolution of west africa over three erosion periods 45 24 24 11 and 11 0 ma recorded by relicts of three subcontinental scale lateritic paleolandsurfaces whose age is bracketed by 39 ar 40 ar dating of lateritic k mn oxides denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early inherited reliefs the subregion underwent low and homogeneous denudation 2 20 m ma 1 over most of its surface whatever the considered time interval this homogeneity is further documented by a worldwide compilation of cratonic denudation rates over long term intermediate and modern cenozoic time scales 10 0 10 7 yr these results allow defining a steady state cratonic denudation regime that is weathering limited i e controlled by the thickness of the lateritic regolith available for stripping steady state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance under such regimes lowering of base levels and their fossilization are primarily imposed by long term eustatic sea level fall and climate rather than by epeirogeny the expression of steady state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated components 9 200 words 13 figures 2 tables

correction to glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle by w r peltier and xianhua jiang

an estimate of motion between the spin axis and the hotspots over the past century 1 estimates of the motion of the spin axis over the past century either neglect plate motion or are relative to the mean lithosphere to understand the response of the spin axis to mass changes it is more useful to estimate spin axis motion relative to the hotspots because the hotspots well represent the solid earth the hotspots originate in the lower mantle which comprises 2 3 of the earth s volume relative to the lower mantle the plates and the upper mantle move in opposite directions to a large extent canceling each other out the mean lithosphere moves quickly relative to the hotspots and poorly represents the solid earth the spin axis has moved relative to the hotspots over the past century along the 68 4 w meridian at 4 03 milliarcsecond yr this motion is 11 counterclockwise of 0 5 milliarcsecond yr faster than and 25 different than spin axis motion relative to the mean lithosphere

the asymptotic response of three dimensional basin offsets to magnetotelluric fields at long periods the effects of current channeling a simple inexpensive numerical algorithm is used to analyze the asymptotic long period behavior of magnetotelluric mt fields in the vicinity of lateral offsets in sedimentary basins the model is based on the distortion or channeling of telluric currents in a horizontal thin sheet although a gross oversimplification of nature the model represents a class of structures which because of excessive computer costs have been relatively unstudied previously within and closely adjacent to the region of the three dimensional 3 d offset significant distortion of the mt parameters occurs skewness coefficients vary from negligible values to over 0 7 principal resistivities vary by an order of magnitude on the other hand there is not a clear correlation between the degree of distortion of the parameters usually evaluated during mt surveys and the magnitude of conventional 3 d indicators e g the skewness coefficient calculations have simulated the technique of averaging resistivity parameters from a large number of field sites in order to arrive at a regionally representative one dimensional 1 d model the results indicate that unless care is taken in adapting the nature of the averaging algorithm to the class of distortions encountered significant bias of the averaged parameters may result these results also suggest that formore this class of structures grave problems may be associated with using the principal resistivity perpendicular to geologic strike the so called transverse magnetic tm mode to infer an equivalent two dimensional 2 d model for the region a 2 d model would likely show significant modulations in the physical character of the basement which are in fact an artifact of telluric distortion caused by current channeling in the surficial heterogeneity 10 figures 4 tables less

informational analysis of apparent earth s resistivity time series to assess the reliability of magnetotelluric measurements abstract we used the informational methods of the shannon entropy power and the fisher information measure to analyze the time dynamics of one year of semi daily values of apparent resistivity calculated on the base of the magnetotelluric mt monitoring at penghu island taiwan the performed statistics indicate that the day time measurements present higher disorder and lower organization than the night time measurements such difference is more evident for the high frequency band of resistivity data which suggests that possibly some source of the electromagnetic field could be of anthropic origin furthermore we also note the characteristics of so called mt dead band with the periodicity of 10 s could be revealed in the high values of shannon entropy power

chemical and physical erosion rhythms of the west african cenozoic morphogenesis the 39 ar 40 ar dating of supergene k mn oxides chemical weathering and mechanical erosion are first order processes of long term tropical morphogenesis which is still poorly deciphered for lack of time constraints we address this issue by laser probe 39ar 40ar dating of generations of cryptomelane k1 2mn8o16 nh2o from the manganese ore deposit of tambao in northern burkina faso this mn deposit results from the supergene weathering of carbonate and silicate mn protores underneath lateritic palaeolandsurfaces it consists of an upper cryptomelane rich domain and a lower domain where pyrolusite b mno2 is the dominant mn oxide the oldest 39ar 40ar ages 59 45 ma are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts 3 4 2 9 ma apparent correlations of 39ar 40ar age groups with d18o and eustatic curves allow definition of the different stages of morphogenesis paleocene eocene ages 59 45 ma bracket a greenhouse period propitious to bauxitic weathering the lack of significant ages between 45 and 29 ma characterizes a period dominated by mechanical erosion during which detrital sediments including lateritic materials were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface two major weathering periods separated by a second erosion episode 24 18 ma are also depicted at the end of oligocene 29 24 ma and lower to mid miocene 18 11 5 ma in the upper domain during which newly shaped land surfaces conspicuously weathered the shorter weathering and erosion episodes recorded in the lower domain from 18 to 2 9 ma led to the final geomorphic changes that were conducive to the formation of glacis the preservation of old cryptomelane 59 45 ma in the upper part of the ore deposit indicates a cenozoic denudation limited to the erosion of previous bauxites and partly of ferricretes

long term stability of global erosion rates and weathering during late cenozoic cooling reports that the geological record of erosion indicates a fourfold increase in global sedimentation rates during the past 5 million years merited a global explanation explanations offered include an increased rate of mountain formation and global cooling now jane willenbring and friedhelm von blanckenburg reanalyse original data sets and use ocean records of beryllium isotope ratios to obtain an indirect measure of past erosion rates and rock weathering they find no clear evidence either for an increase in global erosion or for a pulse of weathered material in the oceans during the past 12 million years they suggest that cenozoic global cooling must have some other cause and the mountain building that did occur had little effect on erosion or weathering flux globally accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates and by inference in erosion rates or weathering fluxes during the past 5 million years here proxy evidence is provided that indicates stable weathering fluxes in the late cenozoic era it is proposed that processes different from an increase in denudation caused cenozoic global cooling over geologic timescales co2 is emitted from the earth s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial this balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1 changes in this balance have been attributed to changes in topographic relief where varying rates of continental rock weathering and erosion1 2 are superimposed on fluctuations in organic carbon burial3 geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1 2 4 widespread observations of a recent 0 5 myr fourfold increase in global sedimentation rates require a global mechanism to explain them4 5 6 accelerated uplift and global cooling have been given as possible causes2 4 6 7 but because of the links between rates of erosion and the correlated rate of weathering8 9 an increase in the drawdown of co2 that is predicted to follow may be the cause of global climate change instead2 however globally rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4 10 moreover proxy records of past atmospheric co2 provide no evidence for this large reduction in recent co2 concentrations11 12 here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13 as evidence we recast the ocean dissolved 10be 9be isotope system as a weathering proxy spanning the past 12 myr ref 14 this proxy indicates stable weathering fluxes during the late cenozoic era the sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean we conclude that processes different from an increase in denudation caused cenozoic global cooling and that global cooling had no profound effect on spatially and temporally averaged weathering rates

reanalysis of ancient eclipse astronomic and geodetic data a possible route to resolving the enigma of global sea level rise predictions of the earth s response to the ice age appear to simultaneously reconcile a set of astronomical geodetic and ancient eclipse observations related to changes in rotation thus ruling out ice melting as a major contributor to 20th century sea level rise we demonstrate that the reconciliation disappears when an improved theory of rotational stability is applied furthermore our reanalysis of longer satellite records renders previous estimates of the secular change in rotation rate suspect the updated ice age predictions and observations permit an anomalous 20th century ice flux of 1 mm yr equivalent sea level rise thus the full suite of earth rotation observations are consistent with a connection between climatic warming and recent melting of ice reservoirs

40ar39ar analysis of supergene jarosite and alunite implications to the paleoweathering history of the western usa and west africa supergene alunite kal sub 3 so sub 4 sub 2 oh sub 6 and jarosite kfe sub 3 so sub 4 sub 2 oh sub 6 are often precipitated during the oxidation of sulfide bearing rocks by meteoric solutions dating of these phases by the sup 40 ar sup 39 ar method allows timing of the progression of the oxidation front during chemical weathering fine scale laser heating sup 40 ar sup 39 ar dating of hypogene alunite and supergene jarosites allows to precisely and accurately time hydrothermal alteration and subsequent supergene oxidation in goldfield nevada the results indicate that pervasive weathering occurred in the western usa during the late miocene approximately 10 ma ago similar application of this technique to the study of weathering and laterite formation in west africa indicates that the last pervasive oxidation event recorded in the weathering profile in this area also occurred in the miocene approximately 13 ma ago the occurrence of a pervasive mid to late miocene oxidation event recorded in these weathering profiles in the western usa and africa and also previously measured in brazil and chile indicates that climatic conditions at that time were conducive to worldwide development of deep weathering sequences subsequent weathering processes have not been as pervasive as the late miocene event indicating a general climatic transition to cooler more drier climates in most of the areas studied the results indicate that deep weathering profiles reflect past climatic conditions and may not be directly linked to the present climates less

variations in the earth s oblateness during the past 28 years 1 analysis of satellite laser ranging slr data indicates that the earth s dynamic oblateness j2 has undergone significant variations during the past 28 years the dominant signatures in the observed variations in j2 are 1 a secular decrease with a rate of approximately 2 75 10 11 yr 1 2 seasonal annual variations with a mean amplitude of 2 9 10 10 3 significant interannual variations with timescales of 4 6 years and 4 a variation with period of 21 years and an amplitude of 1 4 10 10 with minimum in december 1988 two large interannual variations are related to the strong el nino southern oscillation events during the periods of 1986 1991 and 1996 2002 and it appears that another interannual cycle may have started in late 2002 the superposition of the decadal variation on the interannual signal makes the j2 fluctuation appear to be anomalously large during the 1996 2002 period contemporary models of the mass redistributions in the atmosphere ocean and surface water can explain a major part of the 4 to 6 year fluctuations however the cause of the decadal variation remains unknown

detection of a large scale mass redistribution in the terrestrial system since 1998 earth9s dynamic oblateness j2 had been undergoing a decrease according to space geodetic observations over the past 25 years until around 1998 when it switched quite suddenly to an increasing trend that has continued to the present the secular decrease in j2 resulted primarily from the postglacial rebound in the mantle the present increase whose geophysical cause s are uncertain thus signifies a large change in global mass distribution with a j2 effect that considerably overshadows that of mantle rebound

analysis of poorly stratified lateritic terrains overlying a granitic bedrock in west africa using 2 d electrical resistivity tomography abstract two dimensional electrical resistivity tomography has been employed to investigate the subsurface structure of a thick lateritic weathering mantle overlying a granitic bedrock in southeastern senegal the resistivities were measured along two kilometric profiles insuring continuous coverage exploration pits exposed the different weathering layers i e a saprolite a mottled zone a soft ferricrete and a ferricrete whose respective thicknesses were used to constrain the measured apparent resistivity despite their spatial variations colour modulated pseudo sections of apparent resistivity versus pseudo depth including the groundsurface topography clearly show spatial variations in electrical properties of the weathering layers since their apparent resistivity changes faster than their respective thickness the data from a cross borehole survey along with estimates of resistivity for aquifers and granite were integrated into the pseudo sections to provide more useful results about the real resistivity ranges of the weathering layers the resulting geo electrical images document the geometric relations between the different layer boundaries in particular those of the aquifers with the bedrock and groundsurface topographies the spatial relationships between the granitic bedrock and groundsurface topographies suggest that a large part of the actual lateritic weathering mantle is allochthonous this also implies that the actual topography of the bedrock surface was mainly shaped by weathering processes while the hillslope geomorphic patterns result from erosion processes or lateritic weathering of reworked materials leading to ferricrete development according to the different landforms observed it is suggested that climatic changes were implied in the landscape evolution of our study area

global geoid and sea level changes due to present day ice mass fluctuations we predict gravitationally self consistent global geoid and relative sea level rsl perturbations due to present day melting of ice complexes including the antarctic and greenland ice sheets and a suite of mountain glaciers and ice sheets classic analyses of sea level change indicate that these perturbations will depart significantly from eustatic i e geographically uniform trends e g woodward 1888 farrell and clark 1976 although this result has not always been appreciated in modern analyses mass flux of individual ice reservoirs will produce unique geometries of sea level change and this distinctiveness admits the possibility of using global geoid sea surface and rsl signatures of recent climate change to infer the ongoing mass balance of each reservoir rather than simply the net mass flux as an example we show that perturbations to the geoid arising from noneustatic water loads associated with each ice reservoir are sufficiently large at low degrees to be theoretically measurable within 5 years by the grace satellite mission we complete the study by reanalyzing tide gauge data at 23 sites selected by douglas 1997 in a recent analysis of global rsl rise traditionally estimates of global sea level rise are generated by taking the mean of a set of secular tide gauge trends that have been corrected for the influence of ongoing glacial isostatic adjustment gia related to the late pleistocene glacial cycles the common assumption in such studies is that the geographic scatter in the residual gia corrected trends is due to errors in the gia model or unmodeled processes e g tectonics we consider a large suite of gia model predictions and apply a least squares approach to the gia corrected tide gauge trends to estimate the weighting of various present day sea level signatures we find that the fit to the residual rsl trends is significantly improved and that the procedure is able to resolve a long standing observation of anomalously low sea level rates in europe this preliminary analysis which is relatively insensitive to changes in the assumed geometry of the present day mass balance assumes that ocean thermal expansion is globally uniform however the procedure can be easily extended to incorporate a realistic steric contribution once the geometry of the process is sufficiently well constrained

the african landscape through space and time it is generally accepted that cenozoic epeirogeny of the african continent is moderated by convective circulation of the mantle nevertheless the spatial and temporal evolution of africa s basin and swell physiography is not well known here we show how continental drainage networks can be used to place broad constraints on the pattern of uplift through space and time first we assemble an inventory of 710 longitudinal river profiles that includes major tributaries of the 10 largest catchments river profiles have been jointly inverted to determine the pattern of uplift rate as a function of space and time our inverse model assumes that shapes of river profiles are controlled by uplift rate history and modulated by erosional processes which can be calibrated using independent geologic evidence e g marine terraces volcanism and thermochronologic data our results suggest that modern african topography started to develop 30 myr ago when volcanic swells appeared in north and east africa during the last 15 20 myr subequatorial africa was rapidly elevated culminating in the appearance of three large swells that straddle southern and western coasts our results enable patterns of sedimentary flux at major deltas to be predicted and tested we suggest that the evolution of drainage networks is dominated by rapid upstream advection of signals produced by a changing pattern of regional uplift an important corollary is that with careful independent calibration these networks might act as useful tape recorders of otherwise inaccessible mantle processes finally we note that there are substantial discrepancies between our results and published dynamic topographic predictions

late cenozoic increase in accumulation rates of terrestrial sediment how might climate change have affected erosion rates abstract accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents although not everywhere apparently erosion has increased in elevated terrain regardless of when last tectonically active or what the present day climate in many regions sediment coarsened abruptly in late pliocene time sparser data suggest increased sedimentation rates at 15 ma approximately when oxygen isotopes in benthic foraminifera imply high latitude cooling if climate change effected accelerated erosion understanding how it did so remains the challenge some obvious candidates such as lowered sea level leading to erosion of continental shelves or increased glaciation account for increased sedimentation in some but not all areas perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until 3 4 ma when large oscillations in climate with periods of 20 000 40 000 years developed and de

stabilization of large drainage basins over geological time scales cenozoic west africa hot spot swell growth and the niger river reconstructing the evolving geometry of large river catchments over geological timescales is crucial to constraining yields to sedimentary basins in the case of africa it should further help deciphering the response of large cratonic sediment routing systems to cenozoic growth of the basin and swell topography of the continent mapping of dated and regionally correlated lateritic paleolandscape remnants complemented by onshore sedimentological archives allows the reconstruction of two physiographic configurations of west africa in the paleogene those reconstructions show that the geometry of the drainage stabilized by the late early oligocene 29 ma and probably by the end of the eocene 34 ma allowing to effectively link the inland morphoclimatic record to offshore sedimentation since that time particularly in the case of the niger catchment delta system mid eocene paleogeography reveals the antiquity of the senegambia catchment back to at least 45 ma and suggests that a marginal upwarp forming a continental divide preexisted early oligocene connection of the niger and volta catchments to the equatorial atlantic ocean such a drainage rearrangement was primarily enhanced by the topographic growth of the hoggar hot spot swell and caused a major stratigraphic turnover along the equatorial margin of west africa

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

refining the eustatic sea level curve since the last glacial maximum using far and intermediate field sites the eustatic component of relative sea level change provides a measure of the amount of ice transferred between the continents and oceans during glacial cycles this has been quantified for the period since the last glacial maximum by correcting observed sea level change for the glacio hydro isostatic contributions using realistic ice distribution and earth models during the last glacial maximum lgm the eustatic sea level was 125 5 m lower than the present day equivalent to a land based ice volume of 4 6 4 9 107 km3 evidence for a non uniform rise in eustatic sea level from the lgm to the end of the deglaciation is examined the initial rate of rise from ca 21 to 17 ka was relatively slow with an average rate of ca 6 m ka 1 followed by an average rate of ca 10 m ka 1 for the next 10 ka significant departures from these average rates may have occurred at the time of the younger dryas and possibly also around 14 ka most of the decay of the large ice sheets was completed by 7 ka but 3 5 m of water has been added to the oceans since that time

astrometric and space geodetic observations of polar wander optical astrometric measurements of star positions taken during the past century have been recently re reduced using the final hipparcos star catalog in order to determine variations in the mean location of the earth s rotation pole with respect to the earth s crust this newly available polar motion series which is the longest homogeneous polar motion series currently available allows the drift in the pole path to be newly estimated during the 1900 0 to 1992 0 span of the smoothed hipparcos polar motion series the earth s rotation pole is observed to drift at a mean linear rate of 3 51 0 01 milliarcseconds year mas yr towards 79 2 0 2 w longitude this new estimate for the observed trend in the pole path which can be considered to be the present day expression of true polar wander is nearly the same as that estimated in previous studies using the homogeneous ils polar motion series

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

present day secular variations in the zonal harmonics of earth s geopotential we develop a new formalism for the prediction of secular variations in the gravitational potential field of a spherically symmetric self gravitating maxwell viscoelastic planetary model subjected to an arbitrary surface load which may include a gravitationally self consistent ocean loading component the theory is applied to generate the most accurate predictions to date of the present day secular variations in the zonal harmonics of the geopotential the so called for degree arising as a consequence of the late pleistocene glacial cycles in this respect we use the very recent ice 3g reconstruction of the last late pleistocene deglaciation event tushingham and peltier 1991 a comparison of these predictions with those generated using simplified disk models of the ice sheets which have been used in all previous studies of the harmonics 2 indicates that the disk model approximation introduces unacceptably large errors at all spherical harmonic degrees except perhaps 2 predictions have also been made using a eustatic loading approximation also used in previous studies in place of a gravitationally self consistent ocean loading component and we have found that the resulting discrepancy is largest at degrees 2 8 and 10 in the case of the magnitude of the error incurred using the eustatic approximation can be as large as order 10 15 of the predicted value we have attributed this discrepancy to the present day net flux of water away from the equatorial regions arising from the remnant present day adjustment associated with the late pleistocene glacial cycles the effect represents a heretofore unrecognized contribution to the harmonics or alternatively the nontidal acceleration of earth s axial rate of rotation in terms of the latter the maximum anomaly in the length of day is approximately 1 7 s yr we also consider the sensitivity of the data to variations in the radial mantle viscosity profile by using a suite of forward calculations and an examination of frechet kernels the theory required for the computation of those kernels is described herein we find that the radial variation in sensitivity can be a strong function of the viscosity model used in the calculations for models with a uniform upper mantle viscosity um of 1021 pa s forward predictions of the harmonics exhibit a pronounced peak when a wide enough range of lower mantle viscosities lm are considered we denote the lm value at this peak as at the lowest degrees 4 frechet kernels computed for a series of increasing lm values 1021 pa s lm 1023 pa s indicate a migration of the dominant sensitivity of the data to variations in viscosity from regions below approximately 1200 km depth for to regions above this depth in the lower mantle for the sensitivity of the data to variations in the viscosity profile in the shallowest parts of the lower mantle for the case is also reflected in a set of forward calculations described herein as an example predictions made using earth models in which the viscosity above 1200 km depth is constrained to be 1021 pa s do not exhibit the multiple solutions characteristic of the um 1021 pa s calculations the same is true of earth models in which the upper mantle viscosity is weakened an order of magnitude to 1020 pa s the theory described herein is also applied to compute the signal 10 arising from the retreat of small ice sheets and glaciers described by meier 1984 and also from any potential variations in the mass of the antarctic and greenland ice sheets the present day signal due to the late pleistocene glacial cycles dominates the signal from meier s sources at all degrees except 3 in contrast the signal arising from mass variations in the antarctic and greenland ice sheets is potentially comparable to the former a comparison of observational constraints on the data with predictions of the postglacial rebound signal described in this paper in order to infer mantle rheology cannot proceed until constraints are placed on the present day mass flux of these large polar ice sheets we show that the constraints required are weakest at degrees 2 and 4 finally we outline a potentially important procedure for incorporating predictions of the signal due to the late pleistocene glacial cycles and meier s sources with an observational constraint on the datum to yield bounds on the present day net mass flux from the antarctic and greenland ice sheets a rigorous application of this procedure must wait until observational constraints on are reestablished in the literature

planation bauxites and epeirogeny one or two paleosurfaces on the west african margin mapping of lateritic bauxites over the west african rifted margin and analysis of the geomorphic properties of these bauxites combined with available geological data lead to a discussion of the presence of either two meso cenozoic planation surfaces or a single eocene surface to account for the morphotectonic and paleoclimatic evolution of the guinean landforms at large scale two stepped bauxitic levels are documented ongoing or episodic uplift following gondwana breakup and meso cenozoic climate change are proposed to have allowed the formation and abandonment of an early mid cretaceous surface today preserved as the higher bauxitic level and the setting of an eocene planation surface bearing a second generation of bauxites making the lower bauxitic level the single eocene surface hypothesis requires that paleogene bauxitization preserved large pre existing relief to explain two stepped bauxitic levels of the same age the two surface hypothesis is favored because it would explain rebauxitization of alluvial pebbles of bauxites under the lower lateritic level

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

very long term incision dynamics of big rivers constraining large scale incision dynamics of shield and post rift margin domains is key to understanding the sediment routing system over the overwhelming part of the continental surface based on dated and regionally correlated incision markers from west africa we reconstruct for the first time the entire paleo long profiles of big rivers such as the niger at ca 24 11 and 6 ma as well as the eocene topography those rivers have dissected the results provide boundary conditions and calibration for surface process models and paleodrainage dynamics though spatially and temporally variable incision remained mostly below 10 m my with a mean around 5 m my the spatial stability of both the river outlets and divides imposed maintenance or increasing concavity of the river long profiles through time resulting from spatially contrasted adjustment of river segments bounded by recurrent lithogenic knickzones that persisted since 24 ma drainages evolved preferentially by very slow slope decrease or uniform incision in between the stationary knickzones of evolving amplitude with apparently no relation to base level change therefore knickzone height or position may not simply reflect the transient response of big rivers to base level fall as predicted by stream power incision river models this may also challenge uplift histories of deep continental interiors retrieved from river long profiles inversion relying on such models very slow incision allowed amplification of the hoggar hot spot swell and flexural uplift of the continental margin to be recorded by river long profiles emphasizing the potential of big non orogenic rivers as gauges of dynamic topography

estimates of the regional distribution of sea level rise over the 1950 2000 period abstract topex poseidon satellite altimeter data are used to estimate global empirical orthogonal functions that are then combined with historical tide gauge data to estimate monthly distributions of large scale sea level variability and change over the period 1950 2000 the reconstruction is an attempt to narrow the current broad range of sea level rise estimates to identify any pattern of regional sea level rise and to determine any variation in the rate of sea level rise over the 51 yr period the computed rate of global averaged sea level rise from the reconstructed monthly time series is 1 8 0 3 mm yr 1 with the decadal variability in the computed global mean sea level it is not possible to detect a significant increase in the rate of sea level rise over the period 1950 2000 a regional pattern of sea level rise is identified the maximum sea level rise is in the eastern off equatorial pacific and there is a minimum along the equator in the western pacific and in the eastern indian ocean a g

a model for the influence of local inhomogeneities on the magnetotelluric variations at two van stations in greece abstract the magnetotelluric impedance tensor has been measured for the stations thiva and loannina of the van network in greece a model is suggested that can explain the distortion of the tensor by near surface structure if a 3 d near surface local structure and a 1 d regional one are considered the values calculated for the ratio ew ns are in reasonable agreement with the experimental ones the calculated values for the strike direction are 40 and 15 for thiva and ioannina respectively these values are comparable to the experimental ones of 40 5 and 15 5

first 40ar 39ar geochronology of lateritic manganiferous pisolites implications for the palaeogene history of a west african landscape abstract a pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in west africa using 40 ar 39 ar laser probe analysis on different generations of cryptomelane from fe mn pisolites and their embedding lateritic matrices prior to the 40 ar 39 ar geochronological analysis a detailed petrographical study has allowed the identification of successive assemblages from mn pisolites to fe mn pisolites embedded in mn rich and fe rich matrices respectively the age spectra obtained for pisolites including cores cortices and matrices enables the definition of three age clusters around 56 59 44 47 and 24 27 ma the first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced mn leaching and fe al accumulations due to warmer and wetter climatic conditions in the age interval 56 47 ma that characterizes the tertiary greenhouse effect period propitious to bauxite formation in west africa the absence of 40 ar 39 ar ages between 44 and 27 ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled but could still be favourable to fe and or al oxy hydroxides formation before drier climatic conditions became rather effective at the oligocene period to sustain mechanical erosion rather than chemical weathering processes the age cluster 24 27 ma indicates however a reactivation of the manganiferous lateritic weathering late oligocene the 40 ar 39 ar dating results are discussed in terms of climatic condition changes during the palaeogene that were favourable to the genesis of either al and fe or mn oxy hydroxides in the course of development and evolution of the pisolitic formation the 40 ar 39 ar dating also provides a new highlight to large scale geomorphological patterns of west africa

neogene cratonic erosion fluxes and landform evolution processes from regional regolith mapping burkina faso west africa the regionally correlated and dated regolith paleolandform sequence of sub saharan west africa offers a unique opportunity to constrain continental scale regolith dynamics as the key part of the sediment routing system in this study a regolith mapping protocol is developed and applied at the scale of southwestern burkina faso mapping combines field survey and remote sensing data to reconstruct the topography of the last pediplain that formed over west africa in the early and mid miocene 24 11 ma the nature and preservation pattern of the pediplain are controlled by the spatial variation of bedrock lithology and are partitioned among large drainage basins quantification of pediplain dissection and drainage growth allows definition of a cratonic background denudation rate of 2 m my and a minimum characteristic timescale of 20 ma for shield resurfacing these results may be used to simulate minimum export fluxes of drainage basins of constrained size over geological timescales background cratonic denudation results in a clastic export flux of 4 t km 2 year which is limited by low denudation efficiency of slope processes and correlatively high regolith storage capacity of tropical shields these salient characteristics of shields surface dynamics would tend to smooth the riverine export fluxes of shields through geological time

changes of the earth s rotation axis owing to advection of mantle density heterogeneities polar wander the secular motion of the earth s rotation axis relative to its surface has been studied for many years dynamical arguments1 3 show that polar wander can arise from the redistribution of mass in a plastic deformable earth the rate depending on both the rate of mass redistribution and the rate at which the earth s rotational bulge can readjust to the changing rotation axis here we use a viscosity structure obtained through geoid modelling4 a mantle flow field consistent with tomographic anomalies5 and time dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree two geoid during the cenozoic era we show that the rotation axis will follow closely any imposed changes of the axis of maximum non hydrostatic moment of inertia the resulting path of the rotation axis agrees well with palaeomagnetic results7 with the model predicting a current rate of polar motion that explains 40 of that observed geodetically8

determination of long term changes in the earth s gravity field from satellite laser ranging observations temporal changes in the earth s gravity field have been determined by analyzing satellite laser ranging slr observations of eight geodetic satellites using data spanning an interval of over 20 years the satellites used in the analysis include starlette lageos 1 and 2 ajisai etalon 1 and 2 stella and be c geophysical parameters related to both secular and long period variations in the earth s gravity field including the geopotential zonal rates and and the 18 6 year tide parameter were estimated the estimated values for these parameters are centimeters and s18 6 20 0 1 0 2 centimeters the amplitude and phase for the 18 6 year tide are in general agreement with the effects predicted by the earth s mantle anelasticity the solution accuracy was evaluated by considering the effects of errors in various non estimated dynamical model parameters and by varying the data span and data sets used in the solution estimates for from individual lageos 1 and starlette slr data sets are in good agreement the lumped sum values for and are very different for lageos 1 and starlette the zonal rate determination is limited to degree 6 with the current slr data sets analysis of the sensitivity of the solution for the zonal rates to the satellite tracking data span suggests that the temporal extension of the current slr data sets will enhance the solution of zonal rates beyond degree 6

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

postglacial rebound and sea level contributions to changes in the geoid and the earth s rotation axis summary the rate of change of the spherical harmonic degree 2 component of the earth s gravitational potential coa 20 and polar wander velocity are two signals which are sensitive to late pleistocene deglaciation current changes in sea level and deep mantle viscosity diierent load and earth models have been used in earlier papers to predict the component of these geophysical signals caused by the collapse of the last great ice sheets and recent melting of polar ice caps in this paper we present a systematic analysis of the dependence of the predictions on parameters of the ice and earth model we show that the key parameters of the ice model which govern the predictions are the mass the location of the centre of mass and the midpoint of the deglaciation phase of secondary importance is the length of the deglaciation phase and the mean ice load prior to the last glacial maximum these conclusions enable us to make a more robust inference of mantle viscosity than has been made before allowing for the uncertainties in the model of late pleistocene and present deglaciation as previous authors have shown the lower mantle viscosity is the most important rheological parameter and therefore thecoa 20 observation complements sea level observations which are primarily sensitive to lithospheric thickness and the viscosity of the upper part of the mantle using realistic constraints on the sizes locations and timing of deglaciation of the late pleistocene ice sheets and current changes in polar ice caps the observation of coa 20 is used to infer lower mantle viscosity as a function of the present rate of sea level change if the present rate of non steric sea level change is 1 mm yr 1 and that change has been occurring for less than 1000 years then the lower mantle viscosity satis es log10 glm 21 82 0 15 which is consistent with inferences drawn from recent sea level analyses and con rms other analyses of thecoa 20 observation if the polar wander signal is produced entirely by postglacial rebound and current sea level change no more than 20 per cent of the present contribution to global sea level change comes from greenland the above conclusions also hold if the density discontinuities at 420 and 670 km are modelled as phase boundaries rather than material chemical boundaries

postglacial sea level change on a rotating earth first results from a gravitationally self consistent sea level equation we present and solve a gravitationally self consistent sea level equation which governs postglacial sea level variations on a spherically symmetric self gravitating viscoelastic and rotating earth we find that the inclusion of a glacio isostatically induced rotational excitation can significantly affect previous predictions of both present day sea level rates and postglacial sea level histories which were based on a theory that assumed a non rotating earth model to illustrate we consider present day sea level rates and tide gauge corrections along the us east coast and relative sea level curves in the far field of the late pleistocene ice sheets

twentieth century sea level an enigma changes in sea level relative to the moving crust are associated with changes in ocean volume mostly thermal expansion and in ocean mass melting and continental storage t steric t eustatic t recent compilations of global ocean temperatures by levitus and coworkers are in accord with coupled ocean atmosphere modeling of greenhouse warming they yield an increase in 20th century ocean heat content by 2 1023 j compared to 0 1 1023 j of atmospheric storage which corresponds to greenhouse 2000 3 cm the greenhouse related rate is accelerating with a present value greenhouse 2000 6 cm century tide records going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century we take historic 18 cm century the intergovernmental panel on climate change attributes about 6 cm century to melting and other eustatic processes leaving a residual of 12 cm of 20th century rise to be accounted for the levitus compilation has virtually foreclosed the attribution of the residual rise to ocean warming notwithstanding our ignorance of the abyssal and southern oceans the historic rise started too early has too linear a trend and is too large melting of polar ice sheets at the upper limit of the intergovernmental panel on climate change estimates could close the gap but severe limits are imposed by the observed perturbations in earth rotation among possible resolutions of the enigma are a substantial reduction from traditional estimates including ours of 1 5 2 mm y global sea level rise a substantial increase in the estimates of 20th century ocean heat storage and a substantial change in the interpretation of the astronomic record