neogene stratigraphy and paleoenvironments of china abstract neogene deposits are widely distributed in north and west china whereas in southeast china they are confined to isolated basins the neogene in china is predominated by non marine facies and marine deposits are limited to the leizhou peninsula some islands and continental shelves although miocene brackish water deposits were encountered in xinjiang northwest china and a very brief marine transgression took place in north china near the end of the pliocene non marine neogene deposits in china are largely fluvial and lacustrine in origin and composed of mudstones sandstones and conglomerates with local occurrences of lignites and diatomites the maximum thickness may exceed 2000 m e g the qaidam basin a rich fauna and flora have been found in many localities and the most famous are the middle miocene diatomites at shanwang shandong province with unusually well preserved vertebrate and plant fossils a total of 7 mammal stages have been distinguished in china 5 in the miocene 2 in the pliocene and tentatively correlated with the european stages a major realignment of climatic zones occurred before the paleogene neogene boundary when the paleogene climatic zones with a nw trending broad arid zone across the whole of china was eliminated and the arid zone moved to northwest china presumably indicating the onset of the monsoon circulation system during the last few years the marine neogene has been intensively studied in the leizhou peninsula and the three major sedimentary basins off the south china coast the beibu gulf gulf of tonkin basin the zhujiangkou pearl river mouth basin and the yinggehai basin this sequence is over 3000 m thick ranges from the oligocene to the pleistocene and consists of grey mudstones sandstones and conglomerates interbedded with biogenic deposits such as reef limestones and sandy chalk at least 6 foraminiferal assemblages can be recognized in the three basins the psudohastigerina micra assemblage from the oligocene the globigerina ciperoensis cassigerinella chipolensis assemblage from the lower miocene the globoratalia siakensis orbulina suturalis assemblage from the middle miocene the globigerina nepenthes globorotalia acostaensis assemblage from the upper miocene and or lower plicoene the globorotalia tosaensis globigerinoides extremus assemblage from the upper pliocene and the globorotalia truncatulinoides assemblage from the quaternary the nannofossil succession and larger foraminiferal faunas confirm the ages the tropical nature of the neogene faunas is revealed by the occurrences of large foraminifers and warm water species of other groups the transgression cycles are controlled by tectonic processes during the oligocene and early miocene but later by eustatic movements

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

risk assessment and play fairway analysis in frontier basins part 2 examples from offshore mid norway this article documents the methodology and results of a risk analysis for seven plays in the frontier more and voring basins the play types represent different deep marine systems and span a complete transect from upper slope to basin floor because of the sparse well data coverage typical of frontier basins the play models are seismically driven resulting in high level qualitative risk assessments dividing the plays into low medium and high risk categories the objectives of the risk assessment were both to develop an understanding of the distribution of promising reservoir thickness architecture and net to gross in each play type and further to evaluate the range of uncertainties of the depositional model and the quality of the available database we demonstrate here how the effect of perceptions of risk in constituting a valid play can be evaluated during the ranking process and how the process can be used to systematically screen for future exploration opportunities the workflow is rooted in a risk analysis of the geological models because this represents an integrated interpretation of the geology with respect to reservoir distribution architecture and presence of seal in conclusion the prerequisites for proper qualitative risk assessment of geological models are an awareness of reservoir variability and the range of depositional attributes play risk consists of regional risk elements determined by regional depositional models equally important is the uncertainty introduced by poor or lacking data differentiating between play risk and lack of data improved decision making i e whether to acquire acreage data or both edith m g fugelli received an m sc degree in sedimentology from the university of oslo norway in 1987 she has worked in the norwegian petroleum directorate and statoil and joined bp norge in 2001 where she is currently working as a senior regional geologist her interests are sequence and seismic stratigraphy paleotectonic influences on depositional systems and outcrop to seismic defined reservoir characterization tina r olsen received her m sc degree from the university of copenhagen denmark in 1993 and her ph d from the university of bergen norway in 1996 she joined amoco in 1996 and is currently working as a senior reservoir geologist with bp norge in stavanger norway olsen specializes in sedimentology and stratigraphy and her areas of expertise cover exploration development and production geology on the norwegian continental shelf

late mio pliocene chemical weathering of the yulong porphyry cu deposit in the eastern tibetan plateau constrained by goethite u th he dating implication for asian summer monsoon abstract chemical weathering has provided a potentially important feedback between tectonic forcing and climate evolution of the asian continent although precise constraints on the timing and history of weathering are only variably documented here we use goethite u th he and 4 he 3 he geochronology to constrain the timing and rates of chemical weathering at the yulong porphyry cu deposit on the eastern tibetan plateau goethite grains have u th he ages ranging from 6 73 0 51 to 0 53 0 04 ma that correlate with independent paleoclimatic proxies inferred from supergene mn oxides and loess deposits under variable tectonic regimes and vegetation zones over the southeastern asia this correlation indicates that regional climatic conditions especially monsoonal precipitation controlled chemical weathering and goethite precipitation in a vast area of southeastern asia the goethite ages suggest that the asian summer monsoon was relatively strong from 7 to 4 6 ma but weakened between 4 6 and 4 ma and then significantly intensified from 4 to 2 ma the precipitation ages of goethites collected along a 100 m thick weathering profile decrease with depth and indicate a downward propagation of the weathering front at rates of 53 5 10 8 and 4 8 0 6 m ma during the intervals of 7 4 4 2 and 2 0 7 ma respectively the rapid propagation of weathering front during 4 2 ma was caused by abrupt lowering of the water table which was possibly related to local surface uplift or reorganization of the river systems in southeastern tibet during this period

a paleoclimate interpretation derived from pedogenic clay minerals from the piedmont province virginia abstract oxygen isotope ratios of pedogenic kaolinites were measured to estimate the ambient climate conditions during the formation of a saprolite in the piedmont province woodbridge va kaolinite is the predominant pedogenic clay mineral in this saprolite it is formed mostly from weathering of muscovite and to a lesser extent from weathering of microcline a dioctahedral smectite vermiculite phase formed by weathering muscovite is present in minor amounts the 18o 16o ratios indicate that most if not all the kaolinite formed in a cooler climatic regime in which meteoric waters were more depleted in 18o than most present meteoric waters it is possible that the pedogenic kaolinite formed shortly after or during the pleistocene glacial stage in north america this interpretation is consistent with the age of this saprolite derived from 10be dating methods the oxygen isotopic signatures of these kaolinites are not consistent with an older middle or early miocene age for this saprolite

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

dynamical and observational constraints on tropical pacific sea surface temperatures at the last glacial maximum asynchronously coupled atmosphere and ocean general circulation model simulations are used to examine the consequences of changes in the west east sea surface temperature sst gradient across the equatorial pacific at the last glacial maximum lgm simulations forced by the climap sst for the lgm where the west east sst gradient across the pacific is reduced compared to present produce a reduction in the strength of the trade winds and a decrease in the west east slope of the equatorial thermocline that is incompatible with thermocline depths newly inferred from foraminiferal assemblages stronger than present trade winds and a more realistic simulation of the thermocline slope are produced when eastern pacific ssts are 2 c cooler than western pacific ssts our study highlights the importance of spatial heterogeneity in tropical ssts in determining key features of the glacial climate

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

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the pocos de caldas ring complex minas gerais brazil abstract this contribution presents field evidence mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the upper cretaceous field relations demonstrate that ferralitic bauxitic weathering is confined to well drained hill crest and upper slope settings in the pocos de caldas area bauxite normally rests directly on parent rock stagnant drainage and reducing conditions beneath tropical peat deposits along valley floors and in lower slope situations favour sialitic saprolitic weathering the additional importance of high porosity and permeability is demonstrated by co occurring parent rocks with strongly contrasting response on weathering attack in normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering this results in high porosity giving water and atmospheric gases easy access to the more resistant minerals thus in well drained morphological positions bauxite is generated if the same original magmatic parent rocks are shielded against pervasive weathering attack by pre lateritic metasomatic alteration no comparable early permeability arises a fine grained and dense metasomatic assemblage of kaolinite illite microcline and smectite combined with the annealing of fractures and joints is able to prevent bauxite generation and saprolite is formed instead this metasomatically induced saprolite is significantly richer in illite than normal saprolite bauxite formation results in a strong depletion of mg ca na k si ni co cu zn rb ba sr y and ree sialitic saprolitic weathering leads to a generally less pronounced depletion of essentially the same elements silicon and ni are more efficiently retained than in bauxite mobile elements are mn ca na k mg co zn rb ba sr and ree cerium can be re precipitated under oxidizing local conditions closely associated positive and negative ce anomalies are due to inhomogenously concentrated precipitates in small scale spatial alternation with zones of nearly total depletion a general correlation between the type of laterite saprolite or bauxite and ce retainment or depletion does not exist host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites immobility is exhibited by v pb ga nb zr th and u relict zircon is too rare to explain a seemingly retarded hree loss in saprolites

how old is the asian monsoon system palaeobotanical records from china the recent discovery of monsoon records in early miocene raised a question of the time when the east asian monsoon system initiated a distinguishing feature of the modern monsoon system is its geographic distribution which disturbs the zonal pattern indigenous to the planetary climate system and the appearance of the monsoonal climate pattern in the geological records should signify the onset of the monsoon system here we present the results of a compilation of palaeobotanical and lithological data from 125 sites over china that has revealed two completely different patterns of climate zones the palaeogene pattern with a broad belt of aridity stretched across china from west to east and the neogene pattern with the arid zone restricted to northwest of china which has persisted until today the reorganization of the climate system around the oligocene miocene boundary provides evidence for the establishment of the modern east asian monsoon since then the neogene has witnessed significant variations of the monsoon system including enhancement of aridity and monsoon intensity at about 15 13 my around 8 my and 3 my the new data do not support the onset of the asian monsoon system around 8 my rather the new data led to a hypothesis that the transition to the monsoon climate system in east asia occurred in the latest oligocene

40ar 39ar geochronological constraints on the evolution of lateritic iron deposits in the quadrilatero ferrifero minas gerais brazil weathering profiles overlying the sapecado pico and andaime iron ore deposits quadrilatero ferrifero qf minas gerais brazil reach depths of 150 400 m and host world class supergene iron orebodies in addition to hosting supergene ore bodies of global economic significance weathered banded iron formations at the quadrilatero ferrifero and elsewhere e g carajas hamersley are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth laser incremental heating 40ar 39ar results for 69 grains of hollandite group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the sapecado pico and andaime deposits reveal ages ranging from ca 62 to 14 ma older mn oxides occur near the surface while younger mn oxides occur at depth however many samples collected at the weathering bedrock interface yield ages in the 51 41 ma range suggesting that the weathering profiles in the quadrilatero ferrifero had already reached their present depth in the paleogene the antiquity of the weathering profiles in the quadrilatero ferrifero is comparable to the antiquity of dated weathering profiles on banded iron formations in the carajas region brazil and the hamersley province western australia the age versus depth distributions obtained in this study but not available for other regions containing similar supergene iron deposits suggest that little further advance of the weathering front has occurred in the quadrilatero ferrifero lateritic profiles during the neogene the results suggest that weathering in some of these ancient landscapes is not controlled by the steady state advance of weathering fronts through time but may reflect climatic and geomorphological conditions prevailing in a remote past the geochronological results also confirm that the ancient landsurfaces in the quadrilatero ferrifero probably remained immune to erosion for tens of millions of years deep weathering mostly in the paleogene combined with low erosion rates account for the abundance and widespread distribution of supergene iron manganese and aluminum orebodies in this region

development of east asian summer monsoon environments in the late miocene radiolarian evidence from site 1143 of odp leg 184 abundant radiolarians are preserved in the relatively complete upper miocene section of odp site 1143 three radiolarian zones rn6 rn7 and rn8 are recognized on the basis of diartus petterssoni total range in rn6 6 7 ma variations in the abundance of radiolarians especially pyloniid forms radiolarian flux and species diversity are good proxies of upwelling which similar to today s was likely driven by summer monsoons these radiolarian paleomonsoon proxies indicate that the east asian summer monsoon first initiated close to the middle late miocene boundary at similar to 12 11 ma and reached a maximum strength at similar to 8 24 ma therefore the initiation of the east asian summer monsoon was likely earlier than the first indian monsoon which appeared similar to 8 myr ago c 2003 elsevier b v all rights reserved

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

the middle miocene climatic transition east antarctic ice sheet development deep ocean circulation and global carbon cycling abstract the middle miocene represents a major change in state in cenozoic climatic evolution following the climax of neogene warmth in the late early miocene at 16 ma the early stage of this climatic transition from 16 to 14 8 ma was marked by major short term variations in global climates east antarctic ice sheet eais volume sea level and deep ocean circulation in the later stage from 14 8 to 12 9 ma climatic developments included major growth of the eais and associated antarctic cooling a distinct increase in the meridional temperature gradient large fluctuations in sea level followed by a global sea level fall and important changes in deep water circulation including increased production of southern component water east antarctic ice sheet growth and polar cooling also had large effects on global carbon cycling and on the terrestrial biosphere including aridification of mid latitude continental regions increased stability of the eais after 14 8 ma represents a crucial step in the establishment of late neogene global climate systems what controlled these changes in polar climates and the east antarctic ice sheet deep ocean circulation changes probably played a major role in the evolution and variation in polar climates as they have throughout the cenozoic oxygen and carbon isotopic evidence for warm saline deep water production in the eastern tethyan northern indian ocean indicates that meridional heat transport to the antarctic inhibited cenozoic polar cooling and eais growth during the early middle miocene from 16 to 14 8 ma inferred competition between warm low latitude sources derived from the eastern tethyan northern indian ocean and a cold high latitude source southern component water from 16 to 14 8 ma may have been associated with instability in the antarctic climate and cryosphere reduction of warm saline deep water flow to the southern ocean at 14 8 ma may have decreased meridional heat transport to the antarctic cooling the region and leading to increased production of southern component water these middle miocene climatic and cryospheric changes in the antarctic had profound effects on marine and terrestrial climates as the meridional surface temperature gradient increased boundaries between climatic zones strengthened leading to increased aridification of mid latitude continental regions in australia africa and north and south america enhancing the development of grasslands and stimulating the evolution of grazing mammals

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

timing of weathering induced alteration of manganese deposits in western australia evidence from k ar and 40 ar 39 ar dating under particular climatic conditions tetravalent k mn oxides can be formed during terrestrial chemical weathering of mn ores in this study the results of k ar and 40 ar 39 ar dating of weathering related k mn oxides mainly cryptomelane k ba mn 8 o 16 xh 2 o from supergene mn deposits in the pilbara and peak hill mn provinces nw australia and from scattered occurrences of k mn oxides found in deeply weathered rocks in western australia are presented in ideal circumstances k ar and 40 ar 39 ar ages will reflect the time since the formation of k mn oxides however 40 ar 39 ar and k ar dating of k mn oxides which formed as a result of chemical weathering may be complicated by admixture of mica inherited from bedrock and by mixing of different generations of weathering related k mn oxides to correct for admixture of inherited mica a two stage k ar dating technique was applied some samples of dated cryptomelane had finely banded textures and therefore might represent several generations the age differences between these generations were investigated using microsampling and laser microprobe 40 ar 39 ar dating total fusion age determinations of several groups of growth bands of a late oligocene densely layered cryptomelane from the woodie woodie mn deposit nw australia suggest an averaged growth rate of 0 8 or 0 3 mm m y however the growth rate calculated for much of the band sequence is 1 5 mm m y which is five times greater than for the remaining youngest group of bands 0 3 mm m y thus suggesting uneven growth rates the low growth rate might be the result of very slow growth of cryptomelane and or the presence of hiatuses in cryptomelane precipitation total fusion ages measured on void filling banded and colloform cryptomelane in samples from a vertical profile through a mesalike deposit near horseshoe nw australia show peaks at 52 to 48 44 to 43 40 to 39 35 30 to 29 and 7 ma these results are interpreted as reflecting formation of weathering products in the deposit over a remarkably extended period possibly in an episodic manner related to alternation of more humid and drier climatic periods during the tertiary the results from this study show some evidence of episodic formation of weathering related manganese oxides within the deposits studied although modification of age patterns by local hydrological and geomorphological factors allows only a limited degree of correlation of these periods on a regional scale

chemical weathering of monsoonal eastern china implications from major elements of topsoil major element compositions of 36 bulk samples and 41 clay samples which were obtained from 47 topsoils collected in monsoonal eastern china were investigated with conventional wet chemistry and x ray fluorescence xrf spectrometry respectively based on major element analyses the mobility of major elements and latitudinal distributions of sio2 al2o3 ratio chemical index of alteration cia chemical index of weathering ciw and weathering index of parker wip were analyzed meanwhile the suitability of these chemical weathering indices to topsoils in monsoonal eastern china and its controls were discussed these investigations indicate that na k ca mg and si are relatively depleted while mn p fe and ti are relatively enriched in topsoils of the study area by comparison with their contents in the upper continent crust ucc and that alkali metal na k and alkaline earth metal ca mg elements are generally easier to be depleted from their parent materials than other major elements during chemical weathering the latitudinal distributions of cia ciw and wip show that they are suitable to both bulk and clay samples but sio2 al2o3 is only suitable to clay samples not suitable in bulk ones all these investigations indicate a significant dependence of grain size in major element abundance and latitudinal distributions of sio2 al2o3 cia ciw and wip but parent rock type has little effect on them except its impact on the latitudinal distribution of wip in clay samples the significant grain size dependence probably indicates the presence of unaltered minerals in bulk samples thus we suggest that clay samples are more suitable to investigating chemical weathering of sediments on continents than bulk samples the trivial effect of parent rock type probably indicates a relatively uniform chemical weathering on various parent rocks correlation analyses indicate that climate is the dominant control of chemical weathering of topsoils in the study area and the significant latitude effect indicated by the spatial distributions of chemical weathering indices actually reflect the climate control on chemical weathering of topsoils chemical weathering indices actually reflect the integrated weathering history in the study area besides the dominant control of climate other factors like tectonics parent rock biology landform and soil depth and age might also have some effect on the chemical weathering of topsoils in the study area which needs further research c 2013 elsevier ltd all rights reserved

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

geochronological evidence for pervasive miocene weathering minas gerais brazil ar 40 ar 39 laser incremental heating analyses of 22 individual grains of supergene cryptomelane from three weathering profiles up to 400 km apart in the rio doce valley and barbacena regions at minas gerais brazil show that the formation of weathering profiles in these regions is contemporaneous suggesting a strong weathering event in the middle to late miocene 10 8 ma the preservation of these miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the miocene or that a much thicker weathering mantle was present in the region originally assuming a constant thickness of weathering profiles in the region throughout the tertiary we may calculate weathering front propagation rates of 4 8 m myr 1 during the past 10 ma copyright c 2004 john wiley sons ltd

dating constituent formation in soils to determine rates of soil processes a review abstract rates of soil processes are poorly known one approach to determine them could be through the radiogenic isotopic dating of soil constituents such methods have been recently used to date minerals in soils and weathering profiles thanks to the development of high resolution techniques such as 40ar 39ar u series of micro drilled samples u series by laser ablation and u th he and to the development of suitable methodology expanding application of the techniques to a variety of supergene minerals e g fe and mn oxihydroxides carbonates etc in this paper we identify i the main soil constituents that can be dated through these isotopic methods that are mainly fe oxides mn oxides pedogenic carbonates and organic matter ii the main pedogenic processes that leads to their formation in soils and iii the different soil settings that allow transforming soil constituent ages into rates of soil processes namely growth bands transformation fronts and chronosequences

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

oxygen isotope study of cayenne duricrust paleosurfaces implications for past climate and laterization processes over french guiana abstract oxygen isotopic compositions of weathering goethite hematite kaolinite and gibbsite in duricrust paleosurfaces of the cayenne region northeastern french guiana were investigated samples were microdrilled from different horizons and different pedogenetic microfacies of three well characterized duricrust units of eocene unit 1 and miocene units 2 and 3 age the results provided valuable information on duricrust formation processes and on climate change over french guiana the pristine original isotopic signal suited for reconstruction of past climates appears to be best preserved in the massive duricrust facies middle part of the duricrust in the overlying pisolotic duricrust and in the underlying mottled zone the isotopic signal is affected by subsequent re activation of weathering processes oxygen isotope data from massive duricrust indicate that cayenne duricrusts formed from meteoric waters increasingly depleted in 18o with age the youngest duricrust paleosurface unit 3 formed under humid climate similar to present day while the two older paleosurfaces formed from meteoric paleo water distinctly depleted in 18o reflecting a moonsonal contrasted ancient climate for unit 2 added to the 18o depleted nature of the pre miocene ocean for unit 1 the results further suggest that the major climatic change from tropical wet and dry to present day humid regime recorded in amazonia most likely occurred in the late miocene and that miocene laterization events may be responsible for the secondary bauxitization of the eocene duricrust paleosurface the oxygen isotopic composition of weathering minerals occurring in massive duricrust horizons and in particular in hematite nodules and matrix may be used as a tool to discriminate among the different duricrust paleosurfaces encountered in amazonia and constrain the evolution of past climates

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

genesis and paragenesis of the tetravalent manganese oxides of the australian continent the tetravalent manganese oxide minerals of the weathering crust of the australian continent are grouped into lateritic association ultramafic association carbonate association shale association lacustrine association ancient marine crust association and desert varnish association on the basis of field observations of mineral occurrences although the formation of manganese oxides is controlled by the well known eh ph relationship the specific manganese mineralogy is hypothesized to be influenced by the presence of accessory ions or structural templates characteristic of specific geologic environments

pedostratigraphy and paleomagnetism of a 7 0 ma eolian loess red clay sequence at lingtai loess plateau north central china and the implications for paleomonsoon evolution abstract a 305 m thick loess red clay sequence was discovered recently at lingtai which is located in the middle part of the chinese loess plateau it consists of a complete pleistocene loess soil sequence with a thickness of about 175 m and 130 m of tertiary red clay deposits the red clay sequence at lingtai is the thickest one presently known in the loess plateau field observations show that the lingtai red clay contains over 110 couplets of pedogenic b horizons and horizontal carbonate nodule horizons and that it can be regarded as an extremely thick soil complex paleomagnetic studies of 680 samples suggest that the basal age of the lingtai loess red clay sequence is about 7 05 ma grain size analysis of samples taken at 3 3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess thus suggesting a wind blown origin of the tertiary red clay and continuous atmospheric dust deposition in the loess plateau during the last 7 05 ma the pedogenic characteristics of the paleosols within the pleistocene loess and the b horizons in the red clay suggest that the east asia summer monsoon in the latest miocene may have already been slightly stronger than that during the holocene this implies that at about 7 5 ma bp the tibetan plateau could have been uplifted to a critical height in maintaining the east asia summer monsoon system observations of the loess red clay sequence also suggest that the long term changes in the east asia summer monsoon strength have been nonlinear since the latest miocene

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

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

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