cenozoic lateritic weathering and erosion history of peninsular india from 40 ar 39 ar dating of supergene k mn oxides since deccan traps extrusion 65 ma ago thick weathering mantles have developed over peninsular india on both the western coastal lowland and adjacent plateau separated by the western ghats escarpment manganiferous lateritic profiles formed by supergene weathering of late archean manganiferous protores are exposed on paleolandsurface remnants on both sides of the escarpment petrological and geochemical characterizations of samples from those mn lateritic profiles allowed identifying cryptomelane k mn oxide dated by 40ar 39ar geochronology the ages obtained document major weathering periods ca 53 50 ma and ca 37 23 ma in the highland and ca 47 45 ma ca 24 19 ma and discrete weathering pulses at 9 ma and 2 5 ma in the lowland old ages of the highland 53 50 ma and the lowland 47 45 ma indicate synchronous lateritic weathering across the escarpment at a time the peninsula started to drift across the equatorial belt intense weathering periods at ca 53 45 and ca 37 23 ma are interpreted to reflect the early eocene climatic optimum and the onset of asian monsoon regimes respectively the ages further indicate that most of the dissection of the highland must have taken place after 23 ma whereas the lowland was weakly incised essentially after 19 ma our results also document divergent erosion and weathering histories of the lowland and the highland after the eocene suggesting installation of a dual climatic regime across the western ghats escarpment

40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

element concentration and formation of ore deposits by weathering abstract most important supergene ore or mineral deposits are those of al fe mn cu ni co and residual minerals au pge ree nb w sn cr ti minerals phosphates diamond during earth history directed trends of weathering are observed which depend on the following parameters tectogenesis and morphogenesis and evolution of the atmosphere and the biosphere several at least three cycles of weathering can be distinguished during early precambrian late precambrian paleozoic and meso cenozoic times the weathering cycle of the mesozoic cenozoic times is described here each weathering cycle starts under special world wide conditions which are marked by monotonous and flat relief by greenhouse effect and by an initial lateritic sequence followed by increasing differentiation of the weathering sequences the initial lateritic weathering provokes the main process of rock decomposition developing profiles with a thickness of several tens of metres the vertical and lateral variations of the profiles and therewith the quality pattern of ore deposits mainly depend on 1 morphology and drainage during the initial weathering low drainage activity which causes very thick saprolites with a lower smectitic and an upper kaolinitic zone optimal drainage restricts saprolite evolution favouring the formation of thick box horizons 2 lateritic cementation by chemical displacement and neoformation of textures during a progressive stage of lateritisation 3 mid tertiary to quaternary changes of climate and morphology produce a laterite degradation followed by erosion and or resedimentation accompanied by mainly lateral mechanical and or chemical displacement of element or mineral concentrations the evolution of laterites on parent rocks with preconcentration of certain elements leads to their destruction and reorganisation of geochemical provinces in new supergene element associations by relative or absolute enrichment creating special supergene geochemical provinces

chemical and physical transfers in an ultramafic rock weathering profile part 1 supergene dissolution of pt bearing chromite chemical weathering and supergene dissolution processes of pt bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in new caledonia southwest pacific the chemical distributions of alkaline earth transition metals and precious metals including pt and pd were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites and capped by a mottled zone and a lateritic colluvial nodular horizon chemi cal analyses and mass balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of fe co and mn a segregation of ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite and a depletion of mg ca si al and cr the higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile in such a lateritic environment the pt bearing chromite grains are progressively dissolved and the pt group minerals pgm are released in the weathering mantle with a preferential depletion of pd with regard to pt

low concentration of heavy metals in the yangtze estuarine sediments china a diluting setting abstract an examination of the spatial and temporal distribution of 17 major heavy metals i e fe zn ni mg co mn etc was made in the yangtze estuarine sediments revealed by a number of vibrocores taken from the different sediment zones including delta front prodelta and delta shelf transition the results obtained from the numerous core samples which were also measured for pb 210 and cs 137 show that 1 the silty clay comprising the prodelta facies serves as a depositional sink attracting high concentrations of heavy metals delivered from the river mouth 2 after being normalized to aluminum as a proxy for grain size most heavy metals presented in the prodelta facies have lower concentrations than in the other adjacent sediment zones 3 also after normalization concentrations of most heavy metals in the vibrocore sediments tend to increase up core and 4 sedimentation rates in the study area range from 2 0 to 6 0 cm a hence the vibrocores contain a sedimentary record of metal deposition covering more than 50 years it has previously been assumed that sediments off the river mouth were heavily polluted due to industrialization of the shanghai metropolitan area which peaked about 50 years ago however the low concentrations of heavy metals in the study area before and after normalization do not support this assumption the yangtze estuary is characterized largely by the tremendous runoff and unpolluted sediments derived from the upper drainage basin to constitute a unique diluted setting in which the dispersal behavior of heavy metals from the adjacent industrialized coast is influenced substantially as a result the heavy metals in the study area are obviously lower than those previously determined along the coast

influence of the yangtze river and grain size on the spatial variations of heavy metals and organic carbon in the east china sea continental shelf sediments abstract bulk heavy metal fe mn zn cu pb cd al organic carbon and carbonate concentrations grain sizes and c13 of the organic carbon distributions were studied in sediments collected throughout the east china sea continental shelf and the yangtze river delta the results demonstrated that terrigenous sediments from the yangtze river is a dominating factor controlling the spatial variations of heavy metals and organic carbon concentrations on the east china sea continental shelf in addition grain size and recent anthropogenic influences are also major factors modifying the spatial and vertical variations of heavy metals large spatial variations with a band type distribution of heavy metals grain size organic carbon and carbonate were observed higher concentrations of heavy metal and light c13 of the organic carbon were found primarily in the deltaic and inner shelf sediments the band type distribution generally followed the coastline with little variations in the north south direction away from the delta and inner shelf west east direction most heavy metal concentrations decreased rapidly with the exception of cd where high concentrations of cd were also found in the carbonate rich shelf break sediments coarse grained relict sediments and biogenic carbonate are two primary diluting agents for the fine grained aluminosilicate sediments from the yangtze river with high concentrations of heavy metals unusually high concentrations of cu pb and cd showed both spatially and vertically that more pollution prevention measures are needed in the yangtze river drainage basin in order to prevent further heavy metal pollution of the east china sea inner continental shelf

rate effect experiments on round tipped penetrometer insertion into uniform snow snow penetrometers are being developed to detect stratigraphy indicative of slab avalanches results are presented here from experiments performed to assess the relationship between the velocity of a round tipped penetrometer through uniform snow and the resulting force response the range of velocities used is commensurate with that used to drive a manually driven penetrometer through the snow a characteristic spike is noted in profiles during the initial stage of deformation which appears to be more pronounced for the harder snow layers tested an investigation of snow deformation using video analysis of a split axis rod deforming snow against a clear plastic window indicates that the spike in the force signal corresponds to the development of a compaction zone below the penetrometer tip once shear forces that develop in the compacted snow are overcome deformation proceeds in a steady state and there is little or no relationship between the force response and velocity over the velocity range of our experiments

petrologie du gisement lateritique manganesifere d azul bresil at azul state of para brazil the lateritic weathering of organic shales with rhodochrosite and phyllosilicates mainly muscovite has led to one of the largest maganese deposits of brazil 45 mt of reserves with 42 65 mn

cone penetration testing cpt in antarctic firn an introduction to interpretation commercial cone penetration testing cpt equipment was adapted to allow penetrative testing in hard polar firn to depths of 10 m the apparatus is hydraulically driven rate controllable and able to penetrate firn with a resistance of 10 mpa it can be mounted on many types of typical polar vehicles requiring connection to only hydraulics and 12 v electricity data recorded include both cone tip resistance and sleeve friction a parameter not previously examined through such testing this paper describes the development and calibration of the equipment and examines factors including snow density penetration rate and cone size and shape that are shown to affect cpt interpretation cpt can be used efficiently in polar environments to potentially provide estimates of physical parameters in hard firn to substantial depth

a brief introduction to cone penetration testing cpt in frozen geomaterials the cone penetration test has been successfully used to classify soil for 100 years however it has received only limited contemporary use in frozen geomaterials the historical and contemporary use of the cone penetration test in various frozen geomaterials is considered here and contemporary data from recent work in polar snow are examined it is probable that many material physical properties e g density strength and microstructure can be obtained directly from cone penetration testing it appears under utilized as a contemporary scientific and engineering investigative tool in frozen geomaterials

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

latest eocene early oligocene climate change and southern ocean fertility inferences from sediment accumulation and stable isotope data abstract the earliest oligocene 33 5 ma is marked by a major step in the long term transition from an ice free to glaciated world the transition characterized by both cooling and ice sheet growth triggered a transient but extreme glacial period designated oi 1 high resolution isotope records suggest that oi 1 lasted for roughly 400 000 yr the duration of magnetochron 13n before partially abating and that it was accompanied by an ocean wide carbon isotope anomaly of 0 75 one hypothesis relates the carbon isotope anomaly to enhanced export production brought about by climate induced intensification of wind stress and upwelling particularly in the southern ocean to understand how this climatic event affected export production in the southern ocean biogenic silica opal and carbonate accumulation rates were computed for the sub polar indian ocean using deep sea cores from odp site 744 kerguelen plateau our findings suggest that net productivity in this region increased by several fold in response to the oi 1 glaciation in addition calcareous primary producers dominant in the late eocene were partially replaced by opaline organisms suggesting a trend toward seasonally greater surface divergence and upwelling in this sector of the southern ocean we attribute these changes to intensification of atmospheric oceanic circulation brought about by high latitude cooling and the appearance of a full scale continental ice sheet on east antarctica higher terrigenous sediment accumulation rates support the idea that wind induced changes in regional productivity were augmented by an increased supply of glacial dust and debris that provided limiting micro nutrients e g iron rich dust particles we speculate that the rapid changes in biogenic sediment accumulation in the southern ocean and other upwelling dominated regions contributed to the ocean wide positive carbon isotope anomaly by temporarily increasing the burial rate of organic carbon relative to carbonate carbon the changes in burial rates in turn may have produced a positive feedback on climate by briefly drawing down atmospheric pco2

mineralogy geochemistry and genesis of the post gondwana supergene manganese deposit of the carletonville ventersdorp area north west province south africa abstract the post gondwana period is characterized by some of the unique geologic events which shaped the present geomorphological landscape in various parts of the world one of the most important and unique features associated with this geologic period is the formation of high valence state mn4 manganese oxides in weathering crusts of uplifted landscapes underlain by manganiferous protores the study of these manganese oxide minerals has proven not only to be industrially beneficial but they also provide insights into the evolution of the paleo surfaces especially in tropical regions here we present new results on mineralogy geochemistry and genetic model of the north west manganese deposit in the general nice lease area the general nice ore deposit represents a near surface accumulation of supergene manganese wad and nodule deposit in a weathering crust underlain by manganiferous dolostones of the transvaal malmani subgroup the ore formation is attributed to in situ dissolution leaching and surficial weathering of the underlying manganiferous dolostones of the malmani subgroup and carbonaceous protore of the waterval saprolite in a typically oxidic lacustrine setting the combined processes led to the release and mobilization of mn rich colloidal particles sols into the overlying water column of the depository which later precipitated as manganese nodules upon interaction with the alluvial sediments which acted as substrates and centers of accretion mineralogical studies conducted on both scanning electron microscope sem eds and x ray diffraction xrd indicated the predominance of the manganese oxides romanechite cryptomelane and galaxite as major manganese oxide phases and pyrolusite vernadite occurring in minute quantities diagenetic features indicate alteration of pyrolusite and its replacement by romanechite this is evident from the romanechite veinlets cross cutting the fractured groundmass of pyrolusite galaxite appears to represent the latest stage of authigenic mineral precipitation and occurs mostly around the pore spaces other mineral components include hematite goethite clays muscovite silica and inclusions of ilmenite and detrital zircon these accessory minerals occur either in the form of nuclei grains or inclusions in manganese oxide cement

gold a tracer of the dynamics of tropical laterites laterites have developed essentially from long term exposure of cratonal rocks to the atmosphere and hydrosphere the properties of gold density and relative insolubility make residual gold particles tracers of surficial processes causing mineralized laterites gold particles are useful in distinguishing between in situ chemical weathering and physical transport in african lateritic systems developed on slowly eroding stable landforms in an equatorial rainforest setting chemical dissolution and translocation of fine particles are the dominant processes episodic alluvial and or colluvial transport appears to contribute to laterite formation in the subsahelian steppe the use of gold particles coupled with the examination of in situ 10 be from auriferous quartz indicates the applicability of such methods for examination of lateritic weathering dynamics of the tropical transcontinental lithosphere

first successful 40ar 39ar dating of glauconies argon recoil in single grains of cryptocrystalline material potassium argon dating of glaucony one of the few widely occurring authigenic materials in the sedimentary record has provided a large percentage of the dates used for the calibration of the geologic time scale this material has been considered unsuitable for 40 ar 39 ar dating because of the large losses of neutron induced 39 ar during or subsequent to irradiation of samples we have measured recoiled 39 ar and 37 ar from single grains of glaucony 100 350 m in diameter irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation the observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains when the microampoule technique is used the integrated 40 ar 39 ar ages and their associated precision compare very favorably with conventional k ar results because glauconies are inherently heterogeneous populations owing to their complex evolution it is anticipated that single grain 40 ar 39 ar dating may lead to significant refinements in the geologic time scale especially for the cenozoic

neogene weathering and supergene manganese enrichment in subtropical south china an 40ar 39ar approach and paleoclimatic significance deep weathering profiles and associated supergene mn oxides deposits are widely distributed in south china however data on their ages are not available until most recently ar 40 ar 39 laser incremental heating analysis of potassium bearing supergene mn oxides collected from four weathering profiles in the qinzhou fangcheng mn belt guangxi province south china provides for the first time numerical constraints on timing of weathering and supergene mn enrichment thirty eight of 42 cryptomelane grains yield well defined plateau 36 grains or pseudo plateau 2 grains ages ranging from 17 6 0 5 ma to 4 71 0 08 ma 2 sigma the results when combined with data from mn oxides deposits from the adjacent guangdong and hunan provinces document a protracted history of weathering and supergene mn enrichment spanning from the earliest miocene to the latest pliocene in south china this indicates that warm and humid climatic conditions conducive to intense weathering and secondary mineralization prevailed across south china during the whole neogene the climatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and fauna associations from cenozoic inland and marginal sea basins of south china the weathering geochronology data also provide temporal constraints on the initiation and intensification of the east asian monsoon that has been the cause for the south china s humid climate c 2007 elsevier b v all rights reserved

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

weathering history and landscape evolution of western ghats india from 40 ar 39 ar dating of supergene k mn oxides laterites preserved on both sides of the western ghats escarpment of peninsular india have formed by long term lateritic weathering essentially after india seychelles continental break up following deccan traps emplacement c 63 ma ago supergene manganese ores of the western ghats were formed on late archean manganese protores among mn oxides composing the ores cryptomelane k rich mn oxide was characterized and dated by 40ar 39ar geochronology measured ages complement those previously obtained in other south indian manganese ores from the hinterland plateau bonnet et al 2016 and further document three major weathering periods c 53 44 ma c 39 22 ma and c 14 10 ma the later being documented for the first time in india these periods coincide with global paleoclimatic proxies and date the lateritic weathering of three successive paleolandscapes of the western ghats that evolved under slow denudation c 8 m myr over the last 44 myr and were mostly incised during the neogene 22 ma that indicates the western ghats are a relict of a south indian plateau preserved at the headwaters of very long east flowing river systems and above the western ghats escarpment topography and denudation history of this landscape do not require neogene tilt of the peninsula as recently proposed

rate controlled cone penetration tests in permafrost cone penetration tests cpts were carried out in the summers of 1999 and 2000 in a permafrost mound near umiujaq in northern quebec canada to study the cryostratigraphy and assess the creep behaviour of permafrost a new linear pushing system using an actuator technology was specifically designed and developed to accurately control the penetration rate of the cone in the permafrost this system has a load capacity of about 113 kn at a maximum penetration rate of 1 5875 cm s and a stroke of 120 cm and can provide constant penetration rates as low as 4 10 6 cm s it can be disassembled in pieces for easy hand transport and use in remote locations two different types of cpt were performed in the permafrost mound stratigraphic profiling and creep test the first type is a quasi static cpt at a penetration rate of 0 1 cm s providing a stratigraphic profile of permafrost in terms of the measurement of penetrometer sensors as a function of depth the second type is a series of quasi static cpts at incre

supergene manganese ore records 75 myr long campanian to pleistocene geodynamic evolution and weathering history of the central african great lakes region tectonics drives climate assists abstract the southeastern part of the democratic republic of the congo locally hosts proterozoic manganese deposits the deposits of kisenge kamata are the most significant but manganese ores are also known to occur at kasekelesa former katanga province and mwene ditu former kasai province for the present study cryptomelane rich samples from these two localities were dated using the 40ar 39ar step heating method with a co2 laser probe the ages obtained are within a range of c 77 myr to c 2 myr cryptomelane formation took place at c 76 4 ma c 59 6 ma c 45 ma c 35 ma c 23 8 ma c 15 4 ma and c 13 3 ma at kasekelesa and it occurred at c 35 ma c 22 4 ma c 15 ma c 5 5 7 2 ma c 3 6 ma and c 2 1 2 3 ma at mwene ditu the campanian age c 76 4 ma recorded at kasekelesa is the oldest 40ar 39ar age that has up to now been recorded for mn ores from africa it documents the formation of oxidized ore along a campanian or older erosion surface which could be part of the african erosion surface the complete age record suggests that continent wide tectonics accounts for most of the recognized supergene ore formation episodes controlled by vertical lithospheric movements that are ultimately responsible for alternating stages of landscape stability and erosion tectonics is thus regarded as the first order control for secondary ore formation in central africa over the last 80 myr climate is a second order control because sufficient water supply is needed for supergene enrichment whereby climatic conditions are recognized to have been favourable during some relatively cold late mesozoic and paleogene periods as well as during some humid and warm neogene stages

mineralogical characterization of the nkamouna co mn laterite ore southeast cameroon the nkamouna property is an oxide laterite deposit developed on serpentinized peridotite in southeast cameroon it is enriched in co and mn has sub economic ni grades and will be mined primarily for co the ore zone is ca 10 m thick and comprises the lower breccia 3 m thick and ferralite 7 8 m thick units sandwiched between an 8 m thick ferricrete overburden and a barren hydrated mg silicate saprolite the ore mineral assemblage includes mn oxyhydroxides magnetite maghemite ferritchromite goethite hematite kaolinite and gibbsite lithiophorite is the most common mn mineral and is the main host of co mn and a significant proportion of ni it occurs as coatings in pores and on other mineral grains and as concretions and impregnations in the matrix it is invariably associated with gibbsite in the lower breccia and with magnetite and ferritchromite in the ferralite although ore in the lower breccia is volumetrically less important than the ferralite it has the highest grade and co ni ratio the lithiophorite in the ore zone is authigenic and its formation was enhanced by influx of al3 from the overlying ferricrete magnetite and ferritchromite in the ferralite are relicts and contributed to mineralization by enhancing the permeability of the ferralite and providing substrates for the precipitation of the mn oxyhydroxides the structure and mode of occurrence of the lithiophorite makes nkamouna ore amenable to physical beneficiation producing a concentrate with co grades 2 3 4 5 times higher than the run of mine ore

geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in central africa a simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data the lateritic weathering mantle of the haut mbomou area in central africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite a mottled clay layer a soft nodular layer a soft ferricrete and a ferricrete in which kaolinite gibbsite goethite and hematite occur in various quantities incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles instead that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern the respective relations of the silica iron and aluminum balances and the al substitution rate of the hematite on the one hand and of rhg rhg 100 hematite hematite goethite and the kaolinite on the other hand to the consumption or the release of protons h permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles the al substitution rate of the fe oxyhydroxides varies according to the nature of lateritization processes e g saprolitic weathering and aggrading ferruginization vs degradation the observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought this suggests that the actual lateritic weathering mantle of the haut mbomou area may result from different stages of weathering and erosion during climatic changes

long term tropical morphogenesis of new caledonia southwest pacific importance of positive epeirogeny and climate change extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland new caledonia is used to evaluate long term morphogenesis of the island since its emergence in the oligocene the results support the interpretation of the lateritic levels as abandoned stepped remnant landsurfaces that may be grouped into eight island scale erosion levels recording episodic lowering of base level the separation of the erosion levels results from the combination of positive epeirogenic movements of the new caledonia continental ridge and long term neogene climate variability which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events

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

assessing mass balance with the cone penetration test this letter briefly outlines why a friction sleeve equipped cone penetrometer may be a useful tool for assessing surface mass balance of polar ice masses

application of 40ar39ar laser probe and step heating techniques to the dating of diagenetic k feldspar overgrowths abstract the feasibility of determining the age of diagenetic k feldspar overgrowths utilizing 40 ar 39 ar step heating and laser probe dating techniques was tested on samples from the lower cretaceous arkoses of the angola offshore basin a previous study of these samples combining cathodoluminescence and k ar dating indicated that the k feldspar diagenetic overgrowths were 98 16 ma and the detrital cores were 516 26 ma conventional 40 ar 39 ar step heating of overgrown grains from the bulk k feldspar fraction failed to unambiguously distinguish detrital from diagenetic ages similarly step heating analysis of overgrowth concentrates 84 diagenetic did not yield any distinct plateau age attributable to the diagenetic k feldspar detrital and diagenetic k feldspars from the arkoses studied appear to release their ar over similar ranges of temperature partial laser fusion of individual overgrown grains using lowenergy short duration laser pulses carefully focussed on the overgrowth yielded mixed ages with ar contributed from both the overgrowth and the core heat conduction calculations indicate that conductive heating through the core overgrowth boundary is not responsible for partial degassing of the core rather beam scattering along intragrain structural defects and incipient melting of the core must occur a more successful approach involved total laser fusion of individual grains from the overgrowth concentrates which were physically separated from their detrital core one limitation of this approach was that because of the young age and the small size of the diagenetic feldspar simultaneous fusion of several grains was required for accurate age determination our results suggest that future investigations will greatly benefit from the use of on line cathodoluminescence capability to reduce the possibility of contamination by detrital material and the use of a laser beam whose wavelength is strongly absorbed by diagenetic k feldspar

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

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

climate control on early cenozoic denudation of the namibian margin as deduced from new thermochronological constraints abstract the processes that control long term landscape evolution in continental interiors and in particular along passive margins such as in southern africa are still the subject of much debate e g braun 2018 although today the namibian margin is characterized by an arid climate it has experienced climatic fluctuations during the cenozoic and yet to date no study has documented the potential role of climate on its erosion history in western namibia the brandberg massif an erosional remnant or inselberg provides a good opportunity to document the cenozoic denudation history of the margin using the relationship between rock cooling or exhumation ages and their elevation here we provide new apatite u th sm he dates on the brandberg inselberg that range from 151 12 to 30 2 ma combined with existing apatite fission track data they yield new constraints on the denudation history of the margin these data document two main cooling phases since continental break up 130 myr ago a rapid one 10 c myr following break up and a slower one 1 2 c myr between 65 and 35 ma we interpret them respectively to be related to escarpment erosion following rifting and continental break up and as a phase of enhanced denudation during the early eocene climatic optimum we propose that during the early eocene climatic optimum chemical weathering was important and contributed significantly to the denudation of the namibian margin and the formation of a pediplain around the brandberg and enhanced valley incision within the massif additionally aridification of the region since 35 ma has resulted in negligible denudation rates since that time

a new instrument for determining strength profiles in snow cover thermo resistographe realise a l aide d une jauge de contrainte et un systeme d acquisition de donnees microprocesseur z80

manganite formation in the first stage of the lateritic manganese ores in africa abstract lateritic profiles on manganiferous rocks from two areas in central west africa permit the observation of manganite formation on rocks of different primary nature in an area of gondites and tephroitites ziemougoula ivory coast manganites are found to replace tephroites and manganocalcites whereas in a weathered carbonate sequence moanda gabon manganite replaces rhodochrosite in both areas manganite occurrence is limited to a very narrow and basal portion of the altered profile in conformity with the limited stability field established for this mineral it is found that manganites of different mineral lineages may be distinguished chemically from one another by variation in the ratio mno cao mgo

comparative evolution of the senegal and eastern central atlantic basins from mineralogical and geochemical investigations cretaceous to palaeogene sedimentary series in two long continously recovered boreholes from the senegal coastal basin and adjacent cape verde deep sea basin have been compared by mineralogical geochemical electron microscope and microprobe investigations the transition between the diagenetic influences related to the depth of burial and the palaeoenvironmental influences identified in the thick coastal basin successions is complicated by the tectonic instability of the margin during periods of high subsidence rates and crustal thinning as the deep sea basin sediments are only slightly affected by diagenetic changes the comparison between the deep sea and coastal series allows the relative effects of lithostatic pressure tectonics and other palaeoenvironmental factors to be evaluated the disappearance of synsedimentary signatures tectonics climate depositional conditions etc expressed by clay mineralogy occurs below 4 5 km there are no significant diagenetic changes in sediments at less than 2 km depth by comparing identical time slices in coastal and deep sea basins it is possible to recognize from the clay stratigraphic record the main african detrital sources the hot and often semi arid character of the continental climate during the cretaceous the existence of minor but extensive stages of tectonic activity barremian late aptian late albian early cenomanian the temporary existence of semi closed basins on the shelf late cretaceous early palaeogene and the complexity of transport and sedimentation relationships between proximal and distal environments

timing and duration of supergene mineralization at the xinrong manganese deposit western guangdong province south china cryptomelane 40 ar 39 ar dating supergene mn oxide deposits are widely distributed in guangxi guangdong yunnan and hunan provinces south china accounting for 18 of the total mn reserves in the country direct dating of supergene mn enrichment however is lacking in this paper we present high resolution 40ar 39ar ages of mn oxides from the xinrong mn deposit western guangdong to place numerical constraints on the timing and duration of supergene mn enrichment a total of ten cryptomelane samples spanning a vertical extent of 67 m were dated using the 40ar 39ar laser incremental heating technique with seven samples yielding well defined plateau or pseudo plateau ages ranging from 23 48 0 91 to 2 06 0 05 ma 2 one sample yields a staircase spectrum that does not reach a plateau the spectrum however indicates the presence of two or more generations of mn oxides in the sample whose ages are best estimated at 22 34 0 31 and 10 2 0 86 ma respectively the remaining two samples gave meaningless or uninterpretable results due to significant 39ar recoil and contamination by old phases the 40ar 39ar data thus reveal a protracted history of weathering and supergene mn enrichment that started at least in the end of the oligocene or beginning of miocene and extending into the latest pliocene staircase apparent age spectra resulting from banded or botryoidal samples yield an average growth rate of mn oxides at 0 6 0 7 10 3 mm kyr 1 the values indicate that a 1 mm grain of mn oxides may host minerals precipitated during a time span of ca 1 5 m y and accumulation of mn oxides to form economic deposits under weathering environments may take millions of years the distribution of weathering ages shows that the oldest mn oxides occur on the top of the profile whereas the youngest minerals are found at the bottom suggesting downward propagation of weathering fronts however two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations such a complexity of age distribution is interpreted in terms of preferential penetration of mn rich weathering solutions along more permeable fault zones or as a result of multi stages of dissolution and re precipitation of mn oxides a synthesis of geochronological and geological data suggests that formation of the xinrong deposit was a consequence of a combination of favorable lithological climatic and structural conditions because the climatic and structural conditions are similar among the provinces of south china during the cenozoic the geochronological results obtained at xinrong may also have implications for the timing of supergene mn enrichment throughout south china

40ar39ar dating of glauconites measured 39ar recoil loss from well crystallized specimens abstract nine glauconite samples with relatively high k concentrations and which appear to be well crystallized using normal x ray diffraction techniques have been studied using the 40 ar 39 ar method the glauconite 40 ar 39 ar apparent ages exceed their k ar rb sr and in most cases stratigraphic ages by substantial amounts 40 ar 39 ar release spectra sometimes yield plateaus but these apparent ages have no geological significance the results indicate that 39ar is lost by recoil from mineral grains during neutron irradiation consistent with previously reported observations the amount of 39ar loss was measured by isotope dilution for four samples and varied from 29 to 17 in contrast radiogenic 40ar is quantitatively retained during irradiation the very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39ar loss and unsuitable for 40 ar 39 ar dating

china s yangtze estuary i geomorphic influence on heavy metal accumulation in intertidal sediments five intertidal sites along the coast of the yangtze estuary china were examined for concentrations of heavy metals including cu zn pb ni mn and fe in bulk surficial and core sediments differences in heavy metal concentrations are apparent between the sites which are dependent on site specific metal inputs and sediment grain size with the exception of site a shidongkou where the highest concentrations of cu zn pb occur due to direct pollutant input from the nearby sewage outlet heavy metal concentrations at other sites are largely determined by particle size characteristics clay rich sediments together with a downstream location relative to the sewage outlets result in elevated concentrations of heavy metals at site c donghai within the intertidal zone at site c the vegetated upper marsh zone exhibits higher heavy metal concentrations in comparison with the bare mudflat monthly sampling may to september in the scirpus marsh at site c records a temporal variation in heavy metal concentrations both of them can be related to the spatial and temporal variability of sediment grain size a geomorphic understanding of the heterogeneity of sediment grain size is therefore vital to the assessment of sediment pollution in intertidal sediments the present study reveals that the levels of heavy metals found in the yangtze estuary are relatively moderate compared with other estuaries in asia and europe this is related to both the diluting effects of the huge volume of water and sediment in the estuary and a shorter period of industrialisation

geodynamic and climate controls in the formation of mio pliocene world class oxidized cobalt and manganese ores in the katanga province dr congo the katanga province democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves they originated from sediment hosted stratiform copper and cobalt sulfide deposits within neoproterozoic metasedimentary rocks heterogenite the main oxidized cobalt mineral is concentrated as cobalt caps along the top of silicified dolomite inselbergs the supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world class manganese cap deposits in western katanga i e the black earths that are exploited by both industrial and artisanal mining here we provide constraints on the genesis and the timing of these deposits ar ar analyses of oxidized mn ore and in situ u pb sims measurements of heterogenite yield mio pliocene ages the ar ar ages suggest a multi phase process starting in the late miocene 10 5 ma when the metal rich substratum was exposed to the action of meteoric fluids due to major regional uplift further oxidation took place in the pliocene 3 7 2 3 ma and formed most of the observed deposits under humid conditions co and mn caps on metal rich substrata and coeval fe laterites on barren areas these deposits formed prior to the regional shift toward more arid conditions in central africa arid conditions still prevailed during the quaternary and resulted in erosion and valley incision which dismantled the metal bearing caps and led to ore accumulation in valleys and along foot slopes

a petrographic study of phyllic alteration superimposed on potassic alteration the sibert porphyry deposit rhone france zoned wallrock alteration surrounds each phyllic vein three zones distinguished 1 a nearest wallrock zone where the preexisting minerals except quartz are wholly transformed into muscovite 2 an intermediate wallrock zone where the alteration intensity decreases 30 of the rock is replaced by white micas and 3 a farthest wallrock zone where a small proportion of preexisting biotites and plagioclases recrystallizes less than 10 of the primary minerals recrystallize modified journal abstract

micromorphology and formation of pedogenic ooids in calcic soils and petrocalcic horizons abstract ooids pisoids nodules concretions and a host of other terms are used to describe concentrically zoned pedogenic carbonate phyllosilicates silica and other minerals in calcic and petrocalcic horizons worldwide the pedogenic and paleoenvironmental significance of such features is not always easy to interpret because variations in size structure composition and soil geomorphic context have been used to infer distinct modes of genesis for features that appear similar using both optical and scanning electron microscopy petrocalcic soil samples from the jornada la mesa geomorphic surface in new mexico and the mormon mesa geomorphic surface in nevada were studied to reconsider the formation of pedogenic ooids in this paper ooids are 2 mm in diameter and have a range of internal morphologies ooids were especially common within features including clast pendant laminae pisoids and laminar horizons or laminar horizon caps we propose a possible new mode of ooid genesis suggesting that crystal growth can move ooids tiny incremental distances over time and that phyllosilicates are an important genetic component ooids are interpreted to form via 1 mineralization during the evaporation of solutions held by surface tension around mineral grains clasts or petrocalcic fragments with or without the presence of organic matter 2 hydration and plastic behavior of pervasive pedogenic fibrous phyllosilicates that co precipitate with pedogenic calcite and 3 tiny successive movements caused by the crystallization pressure of pedogenic carbonate and other minerals during soil solution evaporation spheroidal morphology is initially promoted by chemical precipitation from evaporating solutions around grains next crystallization pressures from the surrounding matrix displace grains non uniformly promoting stochastic contacts or knocking of ooids against one another and against the soil matrix over long time spans this micrometer scale episodic translocation and rotation in tandem with the plastic behavior of fibrous phyllosilicates enhances the spherical shape of ooids the model described here does not preclude possible biological contributions to ooid genesis nor refute the role of other processes in producing similar features in the same soil finally ooid size is limited by physical and hydrological thresholds pisoids as larger features are more complex and are more likely to involve erosion they do not form in the same manner as ooids this study has implications for paleoenvironmental interpretations of calcic and petrocalcic horizons and for the selection of pedogenic features for isotopic analysis

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

mineralogical micromorphological and geochemical evolution of the facies from the bauxite deposit of barro alto central brazil abstract the hydrolytic alteration of anorthosite from the barro alto stratiform mafic ultramafic complex central brazil caused the formation of an isalteritic bauxite petrological studies using x ray diffraction xrd x ray fluorescence xrf mass balance calculation and micromorphological description via optical microscopy were undertaken to understand the evolution of the bauxitic massif the results suggest the transformation of the isalteritic bauxite f1 into varied alteration facies and their filiations as follows the formation of semi compact massive bauxite f2 and compact massive bauxite f3 by the recrystallisation of gibbsite filing voids resulting in enriched absolute aluminium 9 and 25 respectively the formation of laminar bauxite f4 and fragmentary bauxite f5 which is associated with different types of fragmented massive duricrust such as those formed by mechanical root activity tectonics etc and geochemical dissolution of gibbsite kaolinite neoformation etc processes the formation of palaeopediment bauxite f6 the formation of degraded clay with gibbsitic nodules f7 which is associated with the resilicification process by the oscillation of the water table caused by changes in the shapes of slopes and the cycling of silica by vegetation isalteritic clay f8 facies directly derived from the anorthosite are also found the relationship between the facies reflects the polygenetic evolution of the bauxitic massif

an assessment of 40 ar 39 ar dating of incompletely degassed xenoliths the possibility of measuring the age of eruption of pleistocene lavas by 40 ar 39 ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40 ar which has accumulated in them since their original crystallization the residual 40 ar will increase the apparent k ar age of the xenolith however if a xenolith is of cretaceous age or younger then a plateau in its 40 ar 39 ar age spectrum giving the age of eruption is expected to extend over 25 50 of the total 39ar released if degassing of the xenolith in the magma exceeded 90 and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies if diffusion was from a bimodal population of spheres then the radii must differ by a factor of 10 or more or the diffusion coefficients by a factor of 100 or more or if the spheres were equal in size and in diffusion coefficients then the activation energies must differ by a factor of at least 1 5 that such requirements may be realized in real xenoliths containing k feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early pleistocene basalt flow the experimental results appear to establish that old xenoliths may contain ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption

cenozoic oxidation episodes in west africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies imouraren deposit tim mersoi basin northern niger uranium u deposits can undergo oxidizing events subsequent to which the u may be redistributed or precipitated under distinct mineralogical forms occurrences of u6 minerals can be the result of either i in situ oxidation and alteration of previous u4 minerals or ii direct precipitation from u6 rich solutions samples from imouraren the largest u6 ore deposit known in the world located in the tim mersoi basin northern niger have been studied to determine the nature distribution and timing of occurrences of u and related minerals the host rock is composed of sandstones and analcime rich siliciclastic rocks of jurassic age the u ore is largely dominated by acicular hexavalent u minerals and by a very small amount of uraninite a comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed the petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite documented by textural relationships and chemical analyses using sem and electron microprobe the textural relationship between the u4 and u6 minerals is supported by the near identical ree signatures of the primary uraninite and related secondary u minerals ion microprobe u pb dating from 21 to 1 ma confirms that primary uraninites were altered to u6 minerals under supergene conditions during cenozoic oxidation events in western africa in relation with the well known stages of intense continental weathering that have affected western africa from the early cenozoic to recent times

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

dating of chemical weathering processes by in situ measurement of u series disequilibria in supergene fe oxy hydroxides using la mc icpms abstract constraints on the timing of weathering processes in the northern australian regolith have been obtained by in situ measurement of u series disequilibria in u rich supergene fe oxy hydroxides using a laser ablation mc icpms technique this approach has permitted the measurement of 234 u 238 u and 230 th 238 u activity ratios in finely crystalline fe oxy hydroxides from the ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes 230 th 238 u systematics in the finely crystalline fe oxy hydroxides yield 230 th ages ranging from 60 to 350 ka the most reliable 230 th ages cluster between 122 and 216 ka suggesting fe oxy hydroxides formation and associated weathering peaked during the previous 2 interglacial periods this is supported by the 230 th ages and isotope composition of pisolith fe oxy hydroxide pedogenic nodules cores which are demonstrated to behave as closed systems the u isotopic composition of the fe oxy hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite secondary overprinting is evident in some samples as a large range in 234 u 238 u our results suggest that weathering intensity varies with global climate cycles and that together with weathering events dated by 40 ar 39 ar of mn oxides elsewhere in northern australia feng y x and vasconcelos p 2001 quaternary continental weathering qeochronology by laser heating 40 ar 39 ar analysis of supergene cryptomelane geology 29 7 635 638 weathering rates in this region are orbitally forced

pre quaternary landscape inheritance in australia palaeogeographic reconstructions indicate that parts of the australian continent have been subaerially exposed for hundreds of millions of years some landforms and regolith are demonstrated to be at least 300 million years old but their persistence at or near the surface is inconsistent with long term denudation rate estimates based on cosmogenic nuclides and apatite fission track thermochronology burial and exhumation are suggested as significant preservation factors in conjunction with prolonged tectonic stability and a shift to more arid climatic regimes in the cenozoic copyright 2007 john wiley sons ltd

quaternary continental weathering geochronology by laser heating 40ar 39ar analysis of supergene cryptomelane incremental laser heating analyses of supergene cryptomelane clusters extracted from three distinct weathering profiles from the mary valley region southeast queensland australia yield reproducible and well defined plateau ages ranging from 346 15 to 291 14 ka 2 a precipitation of supergene cryptomelane in this period implies that relative humid climate prevailed in southeast queensland from 340 to 290 ha a result consistent with oxygen isotope analyses of marine sediments from ocean drilling program site 820 and with regional pollen and spore records these results the first report on the precise ar 40 ar 39 dating of quaternary supergene cryptomelane indicate that ar 40 ar 39 analysis of pedogenic minerals provides a reliable geochronometer for the study of quaternary surficial processes useful in the study of soil formation rates continental paleoclimates and archaeological sites devoid of datable volcanic minerals

40ar 39ar and u th he 4he 3he geochronology of landscape evolution and channel iron deposit genesis at lynn peak western australia u th he geochronology of authigenic goethite cements from the lynn peak channel iron deposit cid hamersley province western australia reveals a history of mineral precipitation ranging from ca 33 to 14 ma massive goethites from nearby weathering profiles at roy hill north a possible source of detrital material during the aggradation of the lynn peak channels yield u th he results as old as ca 64 ma the combination of u th he geochronology with incremental outgassing 4he 3he studies on proton irradiated samples reveals that lynn peak goethites host radiogenic 4he in low retentivity lrd and high retentivity hrd domains and that the hrds account for most of the sample mass and have lost very little of their original 4he over geologic time such high retentivity is especially notable given the goethites were collected from the surface where they were subject to significant heating by solar irradiation minor contamination by detrital fragments of potentially 4he rich primary phases e g rutile ilmenite zircon occurs in some samples fortunately the 4he 3he method permits characterization of this extraneous 4he component which is small 10 wt of the total 4he in the goethite and can be corrected out in estimating the goethite formation age these results indicate that the lynn peak channel was already aggraded and undergoing goethite cementation by ca 33 ma the history of aggradation and channel cementation independently measured through 40 ar 39 ar geochronology is consistent with that obtained from the u th he and 4he 3he record laser incremental heating 40 ar 39 ar geochronology of detrital and authigenic mn oxides primarily cryptomelane kmn 8o 16 xh 2o from the same locality in the lynn peak channel reveals that detrital oxides are older than ca 44 ma and as old as ca 65 ma and authigenic oxides are younger than ca 35 ma and as young as ca 16 ma authigenic cryptomelane precipitation and channel cementation occurred throughout the miocene with a particularly strong period at around 20 ma the 40 ar 39 ar geochronological results suggest that regional weathering profiles developed before 44 ma and possibly as early as ca 65 ma were incised and partially eroded in the 44 35 ma interval semi arid conditions promoted the transport of large volumes of sediments aggrading the regional drainage system metasomatic reactions in the aggraded channels caused the ferruginization and in some places manganese replacement of the cids goethite and k mn oxide cementation continued throughout the oligocene and miocene post miocene aridification contributed to the preservation of the cemented channel sediments forming some of the largest readily mineable iron ore deposits on earth

the kinetic friction of snow three components of the kinetic friction of snow are described but only the lubricated component of friction is treated in detail this component depends upon the thickness of water films which support a slider on snow grains over a small fraction of its area the thickness of the film decreases with ambient temperature in a manner which is sensitive to the thermal conductivity of the slider the minimum value of friction at any temperature is reached at an intermediate value of speed because friction decreases as the slider first begins to move and the films form but then increases at higher speeds because of the shear resistance at sub freezing temperatures a small area in the front part of the slider is dry and the friction is high once the water film is formed it increases in thickness towards an equilibrium value which can be very sensitive to slider properties speed and temperature it appears that the mechanisms may be very different for hydrophobic and hydrophilic sliders from the equations derived here it is clear why friction decreases with repeated passes over the same snow

multistage genesis of the late cretaceous manganese karst hosted tasdremt deposit high atlas morocco the eastern part of the souss basin morocco contains several mn deposits in the tasdremt district three mn orebodies occur within the cenomanian turonian dolostones and the senonian coniacian to maastrichtian detrital series the main orebody being located at the boundary between them the mn ores consist of coronadite group minerals mostly coronadite and hollandite in a karstified dolostone new field observations petrographic analyses and geochemical data define the tasdremt deposits as a karst hosted accumulation 11 60 wt mn particularly enriched in ba 1 5 8 2 wt and pb 1 0 5 0 wt with poor contaminations in al fe and p this study shows that the ore forming process is similar to that occurring in the imini c3 level located 100 km to the north east such similarities with the high grade pyrolusite bearing ore suggest that the tasdremt deposit is a lateral equivalent of the imini deposits however the scarcity of pyrolusite in tasdremt results in lower mn grades the tasdremt ores being considered an aborted incomplete system in comparison with the imini deposits 40ar 39ar geochronology of k bearing mn oxides yields late cretaceous ages defining three phases at 91 5 ma 77 5 82 ma and 65 67 ma although the source of metals remains hypothetical mineralizing fluids were carried by o2 free groundwater that mixed with o2 rich shallow meteoric waters at the tasdremt depositional site the dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of mn oxides causing the coeval increase of ph and eh respectively the early atlasic deformation during the late cretaceous is associated with mineralization events and was responsible for creation of low stand reliefs from tasdremt to imini this period enabled karstification and mineralization connecting the tasdremt deposits to other african mn deposits is difficult since the latter consist of laterite resting above paleoproterozoic mn protores and consequently formed under different conditions from karst hosted deposits it is likely that other mn occurrences formed along the atlas belt in similar settings

alteration supergene des protores a grenats manganesiferes dans quelques gisements d afrique de l ouest l alteration meteorique de quartzites a grenat spessartite precambriens conduit sous des climats tropicaux humides a la constitution de minerais oxydes secondaires ceux ci resultent de la superposition dans un ordre variable de divers processus elementaires en particulier hydrolyse des grenats mn ca fe sont elimines et une phase amorphe silico alumineuse subsiste seule neoformation de lithiophorite du manganese importe se combine avec l aluminium des produits amorphes silico alumineux lessivage un vide apparait au centre du grenat altere et s agrandit jusqu a ne laisser subsister que la matrice de quartz piquetee de cavites polyedriques epigenie par la cryptomelane la gangue de quartz est remplacee par la cryptomelane les grenats pouvant rester inalteres

climatic and morphological control of rare earth element distribution in weathering mantles on alkaline rocks abstract in humid coastal regions and in areas of accentuated relief of se bahia brazil weathering of alkaline rocks resulted in the formation of kaolinitic and lateritic weathering mantles under subhumid conditions in the inland region of bahia less mature sandy clayey soil have developed rare earth element ree distribution in the weathering profiles has been investigated in order to establish relationships between climatic and morphological parameters and the enrichment and fractionation of ree under tropical humid climate and intensive drainage conditions ree fractionation is enhanced ce is absolutely enriched in laterite layers while the heavy ree are retained in the saprolite moderate drainage due to high groundwater tables lead to minor fractionation and enrichment of ree restricted drainage conditions in subhumid climatic zones hamper liberation of ree from primary minerals and impede enrichment and fractionation of these elements ree distribution is also controlled by polyphase evolution of the weathering mantle the positive ce anomalies in laterite layers are related to stable mineral phases like mn oxides which have developed in roots of weathering during former less humid weathering cycles the ree fractionation patterns correspond to the drainage conditions which prevailed during the formation of the weathering products thus they may represent a valuable tool for the reconstruction of landscape evolution and the investigation of the paleoenvironment

cone penetration testing in snow the application of the electric cone penetrometer test in snow has been investigated and compared with results from the ram penetrometer test which is normally used for snow profiling and slope stability analysis a special cone penetrometer system was built consisting of a sensitive 1 cm2 electric cone depth transducer and battery operated chart recorder the instruments were tested in april 1985 at three different locations in the swiss alps and the test results were compared with those of the ram penetrometer tests the tests yielded repeatable results up to a depth of 4 m with a high resolution of different snow layers the electric cone tests show more layers than found in the ram profile and snow pit analyses in soft snow the ram resistances appear to be on average about 30 lower than cone resistance values recommendations are given for the future use of electric cone penetration testing in snow key words snow snowpack analysis cone penetration test ram penetrometer cone resistance ram

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

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

a combined u th he and cosmogenic 3he record of landscape armoring by biogeochemical iron cycling u th he geochronology and cosmogenic 3he in iron oxides reveal mineral precipitation ages as old as 55 ma and exposure ages greater than 5 ma for canga cemented plateaus in the quadrilatero ferrifero brazil showing that lateritic profiles overlying banded iron formation bif landscapes in tropical regions have a long history of surface exposure the long term erosion history obtained from cosmogenic 3he on bif plateaus confirms that relic surfaces persist in the landscape for millions of years combined 3he and u th he dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface importantly pebbles of hematite magnetite in colluvia or shallow creeks draining the canga cemented plateaus record a much longer exposure history than in situ canga blocks showing that even older duricrusts now eroded once blanketed these plateaus physically stable but biogeochemically dynamic cangas armor the landscape by pervasive and recurrent iron cycling and cementation slowing down the delivery of weathered bif or friable hematite magnetite ore to erosion

some observations on the interpretation of 40ar 39ar age spectra several recent applications of the 40ar 39ar age spectrum technique have utilized the ability of this technique to resolve the internal ar isotope distribution in crystals for the purpose of placing constraints on the timing of thermal events additionally a few of these studies have attempted to use certain properties of the age spectrum to estimate the temperature and duration of heating experienced by the sample during the disturbance although these successful applications are encouraging the interpretation of age spectra of disturbed samples is in many ways still an art form with the beholder apparently relatively free of constraints that might limit an assessment in fact there are a number of restrictions on interpretation imposed by 1 multiple episodic loss 2 slow cooling 3 mixed phases 4 resolution of the age spectrum 5 excess 40ar 6 recoil artifacts 7 grain distribution and size and 8 phase changes while in the vacuum extraction system these effects are discussed with a view to understanding possible ambiguities in ar release patterns in most cases only with independent geochronological knowledge an understanding of the release systematics and complete sample characterization can these constraints be successfully dealt with in a quantitative framework

manganese deposits of the oudalan province in the northeast of burkina faso and ansongo inlier in eastern mali abstract at least 15 manganese deposits crop out in the west african craton in mali burkina faso ghana and cote d ivoire there average grade estimates range from 17 to 52 total mn oxide and average volume of 5 19 mt of ore they consequently provide an attractive target for exploration and eventual exploitation the tambao and beliata mn oxide deposits in the oudalan province northeast of burkina faso and takavasita agualla and tondibi mn oxide deposits in the ansongo inlier eastern mali generally crop out as a saprolitic residuum that formed during the miocene to eocene and are enriched in manganese to 52 total mn oxide the saprolitic residuum is overlain by blocky to botryoidal manganese rich duricrusts that form part of the resource portfolio the residuum overlies deeply weathered manganese bearing carbonates shales and volcaniclastic siltstones of the birimian supergroup 2 3 2 1 ga that form a low grade resource and likely formed in a heat driven seawater convection system that provided hydrothermal solutions from submarine volcanic centers they are typically intercalated with mafic intermediate volcanic rocks

first 40ar 39ar dating of intense late palaeogene lateritic weathering in peninsular india lateritic surface processes have shaped large platform and cratons of the tropical belt constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long term climatic change and lithospheric deformation weathering histories have been documented for south america africa and australia but precise time constraints of the lateritic weathering processes in south india are still lacking we present 40ar 39ar ages of supergene cryptomelane k mn oxide formed in the sandur mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the mysore plateau and the adjacent deccan traps significant 40ar 39ar ages are estimated between 36 and 26 ma from well defined plateaus in step heating 39ar release spectra and from best fitted inverse isochrones these ages constitute firm time constraints that document intense late eocene oligocene lateritic weathering over peninsular india under the influence of warm and wet climate comparable to that prevailing in tropical humid forests these results imply that southern india was weathered between 36 and 26 ma and may have been dissected mostly in the neogene

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

the development of iron crust lateritic systems in burkina faso west africa examined with in situ produced cosmogenic nuclides we have investigated the development of iron crust laterites on the stable west african craton in northern burkina faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles lateritic soils develop in tectonically stable slowly eroding tropical environments and are a major component of the earth s surface to examine processes affecting laterite formation we determined 10be and 26al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3 8 m myr 1 in addition data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years quartz cobbles presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial colluvial material have10be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode presumably associated with a wetter climate roughly 300 kyr bp these results illustrate the practicality and the potential of the use of in situ produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in situ chemical weathering and mechanical transport

distribution and speciation of heavy metals in surface sediments from the yangtze estuary and coastal areas the concentrations and speciation of heavy metals as cd cr cu hg ni pb and zn in the sediments of the nearshore area river channel and coastal zones of the yangtze estuary china were systematically investigated in this study the concentrations of all heavy metals except ni in the sediments of the nearshore area were higher than those of the river channel and coastal zones in the nearshore area the concentrations of most heavy metals except hg in the sediments of the southern branch were higher than those of the northern branch because of the import of pollutants from the urban and industrial activities around when compared with the threshold effect level tel and geochemical background levels cr ni and as accumulated and posed potential adverse biological effects the speciation analysis suggested that cd pb and zn in the sediments of the three zones showed higher bioavailability than the other heavy metals and thus posed ecological risk significant correlations were observed among cr cu ni and zn r 0 77 in the nearshore area ni cu zn and pb r 0 85 in the river channel and ni cu cr pb and zn r 0 75 in the coastal zone principal component analysis pca indicated that the discharge of unban and industrial sewage shipping pollution and the properties of the sediments contents of fe mn al toc clay and silt dominated the distribution of heavy metal in the nearshore area river channel and coastal zones of the yangtze estuary

episodic weathering in southwestern amazonia based on u th he dating of fe and mn lateritic duricrust abstract this study presents the results of mineralogical analysis and u th he dating of micrometric grains of hematite goethite cryptomelane and hollandite from six samples of bauxite fe mn mn al fe mn al and mn al p duricrusts located in southwestern amazonia brazil the samples were collected at different elevations to constrain temporal epochs of lateritization and to provide insights into the cratonic landscape evolution of proterozoic and paleozoic terrains in brazil three main phases of duricrust formation were recognized indicating a long period of lateritic weathering the first phase preserved as a fragment in fe duricrust on the paleozoic sedimentary rocks began in the permian 290 ma and lasted until the late cretaceous 65 ma the second and best preserved phase formed mn duricrusts on the paleoproterozoic apui lateritic surface 150 250 m a s l and on the aripuana lateritic surface

the age of supergene manganese deposits in katanga and its implications for the neogene evolution of the african great lakes region abstract supergene manganese deposits commonly contain k rich mn oxides with tunnel structure such as cryptomelane which are suitable for radiometric dating using the 39ar 40ar method in africa mn deposits have been dated by this method for localities in western and southern parts of the continent whereas only some preliminary data are available for central africa here we present new 39ar 40ar ages for mn oxide samples of the kisenge deposit in southwestern katanga democratic republic of the congo the samples represent supergene mn oxide deposits that formed at the expense of primary paleoproterozoic rhodochrosite dominated carbonate ores main phases of mn oxide formation are dated at c 10 5 ma 3 6 ma and 2 6 ma for a core that crosses a mineralized interval the latter shows a decrease in age with increasing depth recording downward penetration of a weathering front surface samples of the kisenge deposits also record a c 19 2 ma phase as well as c 15 7 ma 14 2 ma and 13 6 ma phases the obtained ages correspond to distinct periods of paleosurface development and stability during the mio pliocene in katanga because katanga is a key area bordered to the north by the congo basin and to the east by the east african rift system these ages also provide constraints for the geodynamic evolution of the entire region for the mio pliocene the kisenge deposits record ages that are not systematically found elsewhere in africa although the 10 5 11 ma event corresponds to a roughly simultaneous event in the kalahari manganese field south africa the rest of the katanga paleosurface record differs somewhat from records for other parts of africa for which older eocene ages have been obtained this difference is most probably related to the specific regional geodynamic context uplift of the east african plateau with associated erosion and the opening of the east african rift system at c 25 ma are events whose effects in the study area interfere with those of processes responsible for the development of continent wide paleosurfaces

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

geochemical behavior assessment and apportionment of heavy metal contaminants in the bottom sediments of lower reach of changjiang river abstract heavy metal contamination of bottom sediments of the changjiang river is widely reported however the potential source and methods of transportation of these heavy metals in the contaminated sediments is poorly defined this paper examines the correlation between heavy metals and geochemical indices including fe2o3 al2o3 total organic carbon toc and black carbon bc as well as magnetic susceptibility ms using these indices we investigate the contamination characteristics of heavy metals in the sediments by with principal component analysis pca results from 83 sediment samples collected in the lower reach of changjiang river from nanjing to shanghai show that the first principal component accounts for 52 23 of the total variance corresponding to the heavy metals co cr cu ni and zn and conservative components of fe2o3 al2o3 and toc this result indicates that heavy metal distributions are controlled by the transportation and sedimentation of fine particles which is also confirmed by particle size analysis the second principal component explains 24 81 of the variance and is dominated by cd pb and ms which collectively result chiefly from industrial and transportation activities and for ms fly ash production the third principal component accounts for 7 91 of the variance and corresponds solely to hg principal component analysis multiple linear regression pca mlr was used to estimate the contribution of the three principal components to each heavy metal pca mlr results suggest that more than 50 of co cr cu ni and zn were influenced by the particle size effect particle size effect and fly ash account for 37 1 and 27 7 of as cd and pb were mainly explained by fly ash 98 9 of hg was related to pc3 which represented black carbon bc our study indicates that the combination of geochemical and multivariable statistical methods clearly characterizes the geochemistry of heavy metals in sediment of the lower reaches of the changjiang river and suggests that power plants are the main source of heavy metal pollution

quantitative and qualitative insights into bedrock landform erosion on the south indian craton using cosmogenic nuclides and apatite fission tracks due to the scarcity of datable chronostrati graphic markers and the related difficulties of quantifying depths of erosion in cratonic landscapes conceptual models of tropical pediplain and etchplain evolution commonly lack time and vertical elevation scales here we present the first quantitative study of relief development in south india based on in situ produced 10 be dating aided by previously acquired apatite fission track data erosion rates measured on the summits of inselbergs concur with the perception of inselbergs as islands of low erosion but they also reveal a critical contrast between slowly evolving 2 m m y higher relief landforms and more rapidly eroding 8 12 m m y lower relief landforms this relationship is independent of lithology and climate but suggests that erosion rates on the surrounding plain which are controlled by fluvial incision rates into the plateau surface feed back on inselberg dynamism a discussion on the most suitable sites for dating bedrock land forms highlights the problems of sampling design the difficulties of linking measured erosion rates to the minutiae of process and the limitations of both single and multiple chronometer denudation chronologies in low energy cratonic environments

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

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

geochronology of the baye mn oxide deposit southern yunnan plateau implications for the late miocene to pleistocene paleoclimatic conditions and topographic evolution 40ar 39ar laser incremental heating analyses of supergene k mn oxides from weathering profiles at the baye mn deposit southern yunnan plateau sw china were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution weathering profiles in the baye mn mine are dominated by 20 30 m thick saprolites which are locally covered by unconsolidated alluvial deposits we analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations in most cases different grains from the same hand sample and different samples from the same site have reproducible 40ar 39ar plateau ages the plateau ages of individual grains are also consistent with the respective isochron integrated and ideogram ages this age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles sixty four grains from 32 hand samples collected from weathering profiles at four sites a to d yield well defined plateau or pseudoplateau ages ranging from 2 98 0 07 to 0 16 ma 2 the age results show that mn oxides become progressively younger towards lower elevations samples collected from the highest elevation site 1855 m a s l have the oldest ages at 2 98 0 07 to 2 83 0 13 ma samples from intermediate elevations 1821 to 1815 m a s l yield ages ranging from 2 91 0 10 to 2 08 0 11 ma and those from the lowest sites 1768 to 1753 m a s l are younger than 1 25 0 08 ma the age versus elevation relationship indicates fast downward propagation of weathering front since the late pliocene and permits estimating the weathering rates at 3 3 3 8 6 6 1 2 and 11 1 1 9 m myr 1 during the time periods of 2 98 2 83 2 91 2 08 and 1 25 to 0 16 ma respectively the average river incision rates estimated from the surface exposed ages range from 235 10 to 416 19 m myr 1 during the past 3 million years the incision rate calculated from minimum exposed ages at sites b d is 33 2 m myr 1 for the time period of 2 91 1 25 ma which is consistent with the slow incision of streams on the ridges over the yunnan plateau since the early pleistocene three mn oxide pebbles contained in the alluvial deposits overlying the saprolite dominated weathering profiles at the two highest sites provided additional information on the timing of weathering four grains from two pebbles yield plateau ages of 6 32 0 19 to 5 27 0 10 ma whereas the other two grains from the third pebble indicate the minimum formation ages of 8 2 0 4 and 9 3 0 3 ma these ages confirm the existence of older weathering profiles now dismantled in the region manganese oxide 40ar 39ar ages of the baye deposit when combined with results from other localities indicate that lateritic weathering and supergene mn enrichment and by inference warm and humid climates conducive to intense weathering have prevailed over the plateau since the middle to late miocene the climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records confirming that supergene mn oxides can be used as a useful proxy for past climate age clusters of mn oxides at 2 9 2 4 1 2 0 8 and 0 6 0 4 ma are broadly coincident with and thus likely reflect intensification events of the indian summer monsoon that brings moisture and abundant precipitation to the yunnan plateau these clusters also coincide with the periods of significant surface uplift in the yunnan plateau demonstrating a causal link between topographic evolution plateau uplift and intensification of the monsoonal climate

age and evolution of diachronous erosion surfaces in the amazon combining u th he and cosmogenic 3he records u th he geochronology of two weathered plateaus in the carajas mountains para brazil reveals a history of weathering spanning from ca 80 ma to the present for this high elevation 720 m land surface cosmogenic 3he measurements of hematite pebbles and blocks cemented onto the plateaus at two sites n1 and s11d yield erosion rates as low as 0 09 and 0 08 m ma 1 respectively thus these results confirm that the plateau surfaces are nearly immune to physical erosion for tens of millions of years u th he geochronology of ferruginous duricrusts blanketing the low elevation 250 100 m plains surrounding the carajas mountains yield results consistently younger than 10 ma the geochronology results also reveal that the low elevation plain is diachronous becoming progressively younger towards the receding plateaus the spatial distribution of u th he ages permits reconstruction of the history of scarp retreat for the carajas landscape showing that scarp retreat along major river valleys may have been as fast as 20 km ma 1 during tectonically active and humid periods in the cenozoic the cessation of scarp retreat at some sites suggests that metamorphosed banded iron formations and quartzites provide effective barriers to retreating escarpments helping to preserve some of the oldest continuously exposed land surfaces on earth