weathering model for the quantification of atmospheric oxygen evolution during the paleoproterozoic abstract a weathering model has been developed to quantify atmospheric oxygen evolution during the paleoproterozoic the weathering model calculates the concentrations of fe2 dissolved from fe2 bearing primary minerals and oxidized fe3 out of the dissolved fe2 at a given partial pressure of atmospheric oxygen p o 2 during weathering and establishes the relationships between p o 2 and where is the ratio of oxidized and then precipitated fe3 out of the fe2 dissolved from primary minerals to the dissolved fe2 in a whole weathering profile the weathering model considers controlling factors of the redistribution of fe during weathering that is the dissolution rate of fe2 bearing primary minerals the oxidation rate of fe2 and the groundwater flow rate the validity of the model was confirmed by applying the model to the experimental data of olivine dissolution carried out under low o2 conditions the sensitivity analysis of the model has revealed that the formation time of weathering the mineral dissolution rate and the diffusion of o2 into a weathering profile have no or slight influence on resulting in 0 0 and 0 3 changes in log p o 2 caused by four orders of magnitude change of the formation time more than 10 orders change of the mineral dissolution rate and assumed change of the o2 diffusion respectively on the other hand the temperature the ph and the groundwater flow rate have moderate to large effects on 0 6 1 4 and 1 5 changes in log p o 2 for changes of 5 c in temperature 0 5 in ph and one order of magnitude in groundwater flow rate respectively using possible surface temperature ph and groundwater flow rate estimated from the literature we calculated the p o 2 relationships which were then applied to the values of paleosols fossil weathering profiles formed between 2 5 and 1 8 ga taking account of the constraints given by the records of mass independent fractionation in sulfur isotopes and other geological proxies i e 10 6 atm at 2 32 ga and 10 3 atm at 2 0 ga our model implies that the p o 2 levels were 10 9 atm at 2 46 ga 10 4 5 10 2 atm at 2 25 2 0 ga and 10 2 atm at 1 85 ga the present weathering model can constrain p o 2 levels during the possible paleoproterozoic hot periods but not those during the glacial periods when temperature was

paleosols and their relevance to precambrian atmospheric composition various paleosols have been reported from within the witwatersrand and ventersdorp supergroups south africa they were studied in an attempt to constrain the amount of oxygen available in the atmosphere during deposition of the gold and uranium bearing witwatersrand conglomerates the majority of these horizons do not have any physical characteristics of paleosols and none of them have a chemistry consistent with weathering suggesting they have been subjected to modification by later alteration processes a similar chemistry indicating overprinting of any original soil chemistry has been reported from paleosols elsewhere elliot lake region canada hekpoort basalt south africa but it does not appear that significant cognizance has been taken of this fact when using these paleosols to determine the composition of the precambrian atmosphere it is concluded here that characteristics previously attributed to precambrian weathering in an oxygen deficient atmosphere are better explained by post buria

the influence of ambient groundwater discharge on exchange zones induced by current bedform interactions summary we use numerical modeling to investigate the interactions between laminar flow in a water column current bedform induced flow in underlying permeable sediments and discharge of deep groundwater to the water column groundwater discharge reduces the spatial extent of current induced exchange zones within the sediments and may completely prevent the development of current bedform induced flow at high groundwater discharge although exchange zone size is reduced by groundwater discharge the reduction in exchange flux is not significant suggesting reduced residence times of materials cycled through these smaller zones fluid exchange zones tend to be centered around the reattachment points of eddies that also mark the location of flow divides within the sediments while deep groundwater is discharged preferentially along troughs exchange zone depth and the water column reynolds number are functionally related through a morgan mercer flodin model the relationship between the current induced flux through the sediments and the reynolds number is described by low order monotonic polynomial functions

tracing dust sources and transport patterns using sr nd and pb isotopes continental dust transported through the atmosphere can be a tracer of atmospheric circulation and transport as well as being a source of information about climate in the dust s source area this information about the provenance of dust transported from desert areas and deposited elsewhere on the oceans and continents can be evaluated using several observational techniques satellite imagery has been used to this end in recent decades and air mass back trajectories have been calculated to trace dust deposits back to their source area another method consists of analysis of the concentration and composition of certain tracers in the dust itself and comparing them to soils and sediments from possible source areas this method has the advantage of being useable for paleo atmospheric circulation and paleo dust transport studies whereas satellite imagery and air mass back trajectories are restricted to relatively recent times we review here different kinds of isotopic tracers that have been used to trace dust provenance but special attention is focused on the use of sr nd and pb isotopes we evaluate their use in studies of both spatially as well as temporally varying continental dust source and transport

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

precambrian paleosols in canada although at least 38 precambrian paleosols have been reported in canada few have received the attention afforded most other geological phenomena the most intensively studied of these are widely distributed early proterozoic paleosols associated with the sub huronian and matonabbee new name unconformities criteria that have been most useful in identifying precambrian paleosols in canada include macrostructures ascending protolith disruption saprolith and corestones micromorphology sepic texture and peds mineralogical changes dissolution transformation geochemical signatures ascending depletion of some elements characteristic weathering indices and distinctive colour contrast diagenetic mechanical and geochemical overprinting of the paleosols evident in many canadian examples includes paleosol compaction veins cutting pedogenic features and widespread alkali and alkaline earth element metasomatism metamorphism and deformation also tend to mask pedogenic features as attested by th

lower proterozoic red beds evaporites and secondary sedimentary uranium deposits from the east arm great slave lake canada the east arm graben contains a thick sequence of well preserved unmetamorphosed sedimentary rocks three units have been recognized the wilson island group c 2400 2300 ma the great slave supergroup c 2300 1790 ma and the et then group c 1700 1600 ma the wilson island group contains red beds and overlies an apparently normal regolith formed on archaean granites the great slave supergroup contains a normal regolith numerous red beds glauconitic horizions biogenic hydrocarbons and sedimentary uranium deposits there is also abundant evidence for the former presence of evaporites in that part of the sequence that was deposited within 30 of the palaeoequator the uranium deposits originated during low temperature diagenesis by precipitation from oxidizing waters in areas of early accumulated hydrocarbons these observations suggest that the lower proterozoic climate atmosphere and hydrosphere were similar to modern ones and that uranium prospectors in such old terrains should look for chemical deposits as well as the classic placer types

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

advective flow through the upper continental shelf driven by storms buoyancy and submarine groundwater discharge data from monitoring wells on the carolina usa continental shelf require a reevaluation of pore water exchange across the seafloor temperature salinity and 226ra data from 3 offshore wells require rapid and frequent pore water exchange to depths of several meters through sands and other permeable sediments the exchange is driven by tidal pumping storms buoyancy and submarine groundwater discharge tidal pumping causes semidiurnal temperature cycles in one well that can only be explained by rapid fluid flow the effect of large storms is evident in sudden temperature changes 2 4 m below the seabed in two wells these changes require considerable pore water exchange sudden cooling of the ocean may cause pore waters to become buoyant with respect to the ocean and trigger pore water convection to several meters depth as observed in one well the wells also display frequent large factor of ten changes in 226ra activity this requires a substantial input of 226ra enriched fluids to replace 226ra that is lost during exchange events these data support a fourth flow process which is groundwater seeping from the underlying limestone the pore spaces in the overlying sand provide a temporary reservoir for these fluids we conclude that pore water residence times in 2 4 m thick sands on this continental margin are on the order of a few months or less

potassium metasomatism of precambrian paleosols abstract independent of substratum composition ancient regoliths formed by subaerial weathering are characterized by accumulation of k and other alkaline elements potassium enrichment due to the formation of clay minerals is a trend typically observed in regoliths developed by modern hypergene processes however the k content in precambrian paleosols is much higher than in modern weathering profiles k metasomatism affecting buried hypergen targets is a possible mechanism for k accumulation a novel reaction transport dissolution precipitation model designed as part of this work proved able to reproduce many essential features of post burial alterations of a subaerial weathering profile formed on basaltic rocks including potassium enrichment dehydration compaction and pore filling that impacted the buried regolith the phenomena of preservation of the primary basaltic structure the apparent selective alteration of soil levels and immobility of al ti and p could be explained by the model

problematic urn shaped fossils from a paleoproterozoic 2 2ga paleosol in south africa abstract small 0 3 1 8 mm long locally abundant urn shaped fossils within surface horizons of a paleosol in the 2 2 ga hekpoort formation near waterval onder south africa are here described and named diskagma buttonii retallack gen et sp nov the fossils are from fresh rock of a deep highway cutting and have been metamorphosed to upper greenschist facies like their matrix despite metamorphic alteration total organic carbon of the samples was 0 04 and its isotopic composition 13 c was 25 6 0 08 two standard deviations versus vienna pee dee belemnite standard organic outlines of the fossils are also accentuated by recystallized berthierine and opaque oxides the fossils are locally clumped within surface swales of a vertisol paleosol identified from characteristic penecontemporaneous deformation clastic dikes between swales of mukkara structure and from pronounced geochemical differentiation phosphorus and copper strain corrected mass depletion characteristic of an oxidized biologically active soil this paleosol s chemical composition is evidence of temperate humid climate mean annual temperature 11 3 4 4 c and mean annual precipitation 1489 182 mm associated paleosols indicate atmospheric co 2 of 6640 12 880 4293 ppm 0 6 and 0 9 5 atmospheric o 2 the best preserved examples of diskagma are shaped like an urn with a flared rim and closed below the flare observation of hundreds of specimens in thin section reveals substantial variation in growth elongation and decay shredding and deflation they had a hollow ellipsoidal interior that is unusually devoid of opaque debris unlike the matrix diskagma is superficially comparable with lichens such as cladonia ascomycota and geosiphon glomeromycota definitive reproductive structures remain unknown they predate the oldest other likely fossil eukaryotes 1 9 ga and fungi 1 5 ga and current molecular clock estimates for eukaryotes 1 6 ga and fungi 1 1 ga lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks although biological affinities of diskagma are uncertain these fossils reveal the general appearance of paleoproterozoic life on land

tropical laterites life on land and the history of atmospheric oxygen in the paleoproterozoic the ca 2 2 ga hekpoort paleosol of the transvaal supergroup in southern africa has been considered a type example and the youngest iron depleted paleosol formed under a reducing atmosphere in the early precambrian however the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron depleted pallid lower zone and an iron enriched lateritic upper zone previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds the laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass revised stratigraphic correlation indicates that the hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of wolhaarkop in griqualand west this information indicates that the oxygen evolution curve based on loss or retention of iron in paleosols should be reexamined

numerical model on the flow dynamics around the sediment water interface in the tidal coastal area abstract investigating flow dynamics around the sediment water interface is one of difficult issues in marine environment it has important significance to study the physical chemical and biological activities between the overlying water and sedimentary layer with tidal forcing in this study we established a numerical model to investigate the flow dynamics around the sediment water interface in the tidal coastal area the model reflected the hydrodynamics of periodic reciprocating unsteady flow in the overlying water layer and the fully coupled simulation of flow movement between the overlying water layer and sedimentary layer a sedimentary layer an overlying water layer and an air layer were all treated as the fluid zone the unsteady reynolds averaged navier stokes equations and the reynolds stress model with porosity were solved by the finite volume method moreover the drag source term in the momentum equation was modified as the darcy forchheimer extended form the model showed a strong performance in simulating the hydrodynamics of coupled flow around the sediment water interface with tidal forcing in a coastal area with considering the sedimentary layer the benthic boundary layer velocity distributions were more accurate than those obtained by a wall function model around the sediment water interface there was inertial loss in the flow furthermore velocity increased with increasing porosity and velocity gradient became larger compared with models with darcy s law the numerical model in this study had better performance in the turbulent characteristics of sediment water interface layer the model can lead to better understanding of the exchange mechanisms of oxygen nitrogen and nutrient between overlying water and sediment

dust storm activity over the tibetan plateau recorded by a shallow ice core from the north slope of mt qomolangma everest tibet himal region 1 based on a 39 8 m ice core drilled at the east rongbuk glacier 27 59 n 86 55 e 6518 m a s l north slope of mt qomolangma everest central himalaya the changes of dust storm over the tibetan plateau tp recorded by insoluble particle and its possible connection with the winter jfm north atlantic oscillation nao are investigated the results show that the level of dust storm activity exhibits a significant decreasing trend during the past 50 years this may be attributed at least partly to a decrease in wind speed over the tp and its surrounding regions the normalized time series of the winter nao index shows a significant inverse correlation with the mass concentration of insoluble particle implying a possible connection between the winter nao and the dust storm activity over the tp

a 1 25 ga depositional age for the paleoproterozoic mapedi red beds kalahari manganese field south africa new constraints on the timing of oxidative weathering and hematite mineralization the great oxidation event goe is marked by the loss of readily oxidizable detrital minerals and the onset of oxidative weathering one of the oldest post goe weathering surfaces which extends for almost 350 km along strike occurs in griqualand west south africa it is best preserved east of the blackridge thrust where oxidized paleoweathering profiles are developed below the unconformity at the base of mapedi gamagara red beds in the maremane dome the red beds preserve pisolitic hematite laterites which indicate a highly oxygenated atmosphere and suggest hot and humid climatic conditions the mapedi and gamagara formations are undated east of the blackridge thrust but were thought to be lithological correlatives of the 1 91 ga mapedi red bed sequence to the west here we report a u pb zircon age of 1 25 ga for a felsic tuff in red beds of the mapedi formation in the kalahari manganese field the new tuff age shows that the mapedi red beds east of the thrust were deposited 650 m y after the mapedi formation to the west and therefore they are part of a distinct mesoproterozoic sequence based on lithologic and sedimentological similarities the mapedi east and gamagara formations are likely to be correlatives that were deposited on an ancient weathering surface at ca 1 25 ga our findings suggest that key evidence for a highly oxygenated atmosphere during the early paleoproterozoic actually formed at ca 1 25 ga during a major episode of mesoproterozoic oxidative weathering

geochemical evidence for sources of surface dust deposited on the laohugou glacier qilian mountains abstract atmospheric dust deposited on glacier surfaces can decrease snow albedo by enhancing lighting absorption and by forming cryoconites via microbial activity which accelerates glacier melt using snow and cryoconite sampled from the laohugou glacier no 12 in the qilian mountains during spring 2012 and summer 2014 this work investigates the sr nd isotopic and rare earth element ree geochemistry of dust and its environmental significance results demonstrate that dust rees possess lower eu eu and l hree and higher gd yb n values compared with that of the mu us and hobq deserts but yield higher la yb n l hree la sm n and la yb n ratios than that of the taklimakan desert the ree composition of glacier surface dust is similar to the material from nearby arid regions such as the qaidam basin and the tengger and badain jaran deserts surrounding the eastern tibetan plateau the e nd 0 values of glacial dust resemble the isotopic ratios of the qaidam badain jaran tengger deserts and local dust e nd 0 value of 13 6 but differ from that of the taklimakan desert which has higher 87 sr 86 sr ratios analysis of air mass trajectory also indicates a potential dust input from the surrounding areas to the alpine glaciers in the qilian mountains these results strongly indicate that the dust source from the arid northern tibetan plateau region was the dominant contributor to atmospheric dust deposition on the glacier surface rather than dust originating from the long range transport of taklimakan desert material

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

sr nd isotope evidence for modern aeolian dust sources in mountain glaciers of western china in order to apportion the dust sources of mountain glaciers in western china the sr nd isotopic compositions of insoluble particles were determined in snow samples collected from 13 sites the combined plot of 87 sr 86 sr and

advective transport affecting metal and nutrient distributions and interfacial fluxes in permeable sediments abstract our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments leading to specific and reproducible complex patterns of fe mn and nutrients oxygenated water forced into the sediment when boundary flows were deflected by protruding sediment structures generated distinct zones of nitrification and ferric iron precipitation this inflow was balanced by ammonium rich porewater ascending from deeper sediment layers thereby creating an anoxic channel where dissolved fe2 and mn2 could reach the surface between the zones of ferric iron precipitation and fe2 upwelling a layer with increased manganese oxide and solid phase fe ii concentrations formed indicating redox reaction between these components the establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes while the smooth control core was found to be a sink for metals and nutrients the sediment with mounds acted as a source for these substances our experiments show that in sandy sediment with an oxidised surface layer reduced metal species can be released to the water column by flow topography interactions we conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

multiple sources supply eolian mineral dust to the atlantic sector of coastal antarctica evidence from recent snow layers at the top of berkner island ice sheet the sr and nd isotopic composition of dust extracted from recent snow layers at the top of berkner island ice sheet located within the filchner ronne ice shelf at the southern end of the weddell sea enables us for the first time to document dust provenance in antarctica outside the east antarctic plateau eap where all previous studies based on isotopic fingerprinting were carried out berkner dust displays an overall crust like isotopic signature characterized by more radiogenic 87sr 86sr and much less radiogenic 143nd 144nd compared to dust deposited on the eap during glacial periods differences with eap interglacial dust are not as marked but still significant indicating that present day berkner dust provenance is distinct at least to some extent from that of the dust reaching the eap the fourteen snow pit sub seasonal samples that were obtained span a two year period 2002 2003 and their dust sr and nd isotopic composition reveals that multiple sources are at play over a yearly time period southern south america patagonia in particular likely accounts for part of the observed spring summer dust deposition maxima when isotopic composition is shifted towards younger isotopic signatures in the spring possible additional inputs from australian sources would also be supported by the data most of the year however the measured isotopic signatures would be best explained by a sustained background supply from putative local sources in east antarctica which carry old crust like isotopic fingerprints whether the restricted east antarctic ice free areas produce sufficient eolian material has yet to be substantiated however the fact that large 5 mu m particles represent a significant fraction of the samples throughout the entire time series supports scenarios that involve contributions from proximal sources either in patagonia and or antarctica possibly including snow free areas in the antarctic peninsula and other areas as well this also indicates that additional dust transport which does not reach the eap must occur at low tropospheric levels to this coastal sector of antarctica

nd isotopes of siliciclastic rocks from tibet western china constraints on provenance and pre cenozoic tectonic evolution nd isotope data from pre cenozoic siliciclastic rocks from tibet are used to determine variations of source regions with time and hence tectonic history on average the tdm and tchur ages of the tibetan samples are up to 1 83 0 36 ga and 1 25 0 45 ga respectively indicating that tibet is underlain by lower proterozoic basement and has close affinity with the himalayas an nd isotope crisis occurred at the early permian with older samples defining a trend of slight decrease in end values with time while the lower permian rocks sharply increasing 6 5 end units this is related to the coeval widespread basaltic magmatism in tibet which could be responsible for tibet rifting away from northern gondwanaland the central qiangtang meta sedimentary samples have distinct nd isotope characteristics end 0 19 87 1 51 tdm 2 07 0 13 ga tchur 1 78 0 21 ga from the songpan ganzi samples end 0 12 77 2 87 tdm 1 49 0 18 ga tchur 1 08 0 23 ga suggestive that these two groups were derived from differentsourcesandthatthe former werenot theequivalents ofthe latterthat wasunderthrust fromthe jinsasuture andmayrepresent subducted continental crust the mid jurassic flysch rocks in northern lhasa have typical upper crust nd isotopic signatures and could have formed under a passive margin environment their bangong counterparts averagely 6 35 units higher in end values unveil derivations from a likely magmatic arc along the northern bangong margin basaltic pebblewith low nd content and endvalue from the berriasian valanginian rocks in central tibet along with the samples with generally high end values shows that the bangong ophiolites could have been a main source of sediments during the earliest cretaceous time a significant transition of sediment sources occurred at the beginning of the hauterivian stage 120 ma sedimentary basaltic tuff end t 2 27 in the hauterivian lower cenomanian samples in central tibet and common nd signatures for input of mantle like materials in these rocks indicate that mantle derived magmatic rocks could have constituted a significant source the huge hauterivian lower cenomanian sediments in central tibet are thus inferred to have been accumulated under a back arc rifting setting 2007 elsevier b v all rights reserved

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

late archean initiation of manganese metallogenesis its significance and environmental controls abstract manganese metallogenesis was initiated in a modest scale during the late archean time selectively in india and brazil sedimentary deposits occur in both greenstone belts and high grade granulite terranes in shallow water shelf regimes only and are often intimately associated with stromatolites the changes in the rates of continental crustal growth mantle heat flux and outgassing along with the evolution of tectonic regimes sedimentary environments the state of the biosphere and finally the compositions of interacting atmosphere hydrosphere system from the early to the late archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments compared to the spectacular manganese event represented by the giant paleoproterozoic kalahari deposit south africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca 2 5 2 4 ga ago the modest initiation of the late archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin margin oxygen oases as a prologue for the succeeding stratified ocean system the dissolved manganese in the basin water that was oxidized to form shelf type shallow water deposits is suspected by analogy with contemporary banded iron formations bifs but not proved to be of hydrothermal ancestry the initiation of concentrated manganese sedimentation during the late archean by itself provides an evidence for an increase in the oxygen content in the contemporary atmosphere hydrosphere system compared to that in the earlier period supporting the geochemical indications from paleoweathering profiles

geochemistry and mineralogy of the 2 45 ga pronto paleosols ontario canada abstract the ontario canada pronto paleosols which formed around 2 45 ga on archean granite were studied through field observation mineralogical study by microscope and microprobe and chemical analyses of major and 45 trace elements including rare earth elements rees since the first full description of its mineralogy and chemistry of major elements by gay and grandstaff gay a l grandstaff d e 1980 chemistry and mineralogy of precambrian paleosols at elliot lake ontario canada precambrian res 12 349 373 the pronto paleosols are among the most cited paleosols of the archean early proterozoic era though several questions have been raised as to whether they are true in place ancient soils results of this study show that the studied section meets the criteria for being in place paleosols furthermore the profile has no explicit traces of being strongly affected by hydrothermal alteration the ree containing accessory minerals were destroyed through weathering and secondary phosphate minerals with rees formed all of these observations support the weathering origin of the pronto paleosols however the profile should be identified as a saprolith rather than paleosol as even the uppermost samples exhibit the granitic texture of the parent rock the samples in this study were collected mainly from two sites site i and site ii saprolith samples from site i show the retention of fe and increases of the fe iii fe ii ratios and the chemical index of alteration cia values toward the unconformity as for the site ii samples the fe ti ratios show slight decreases with increasing cia values in the upper part of section whereas these samples also show the same trend of increasing fe iii fe ii ratios as the samples from site i microprobe analyses show that fe from the parent rock distributes mainly in mafic minerals as hornblende biotite or chlorite which were completely destroyed during the early stages of weathering in the saprolith section fe is present in sericite aggregates in the form of obscure patches of iron oxide tens of micrometers in size all these characteristics indicate that i the fe was not lost during soil formation at site i and slight amount of fe was lost at site ii and ii ferrous fe was quantitatively transformed to ferric fe i e oxidation during soil formation in the samples from both sites the ratios of al ti and si ti decrease toward the top of the profile together with the destruction of minerals toward the unconformity such as apatite biotite and chlorite the depletion of al and si suggests that the parental granite was weathered by an acidic weathering solution during the archean early proterozoic era the high atmospheric partial pressure of co 2 p co 2 is the most probable candidate for producing the acidic weathering solution

reappraisal of a 2200 ma old paleosol near waterval onder south africa abstract a fossil soil paleosol about 2200 ma old in a deep roadcut between the towns of waterval boven and waterval onder transvaal here called the waterval onder clay paleosol was formerly reported to have formed on the hekpoort basalt it is here thought to have developed on a thick formerly at least 5 5 m upward fining and thinning sequence of sandstone siltstone and shale derived from pre existing soils on hekpoort basalt within the basal portion of the fluvial dwaal heuvel formation in contrast to paleosols developed on hekpoort basalt the waterval onder clay paleosol is little deformed lacks corestones and shows compositional variation and berthierine rich laminae interpreted as relict bedding the paleosol has a ridge and swale topography linear or wavy gilgai and a subsurface structure mukkara typical of modern vertisols since there is considerable relict bedding it probably formed over a short period of time less than 7000 years and was not self plowing to the extent seen in modern vertisols clastic dikes filled with sandstone from the overlying rocks are evidence that the profile was periodically dry to depths of at least 2 m more drab colored rocks below that may represent the level of permanent water table iron and manganese stain of layers and claystone granules within the surface horizon of the profile have some characteristics of modern rock varnish and could be taken as evidence for life in the soil the high ratio of ferric to ferrous iron near the surface of the paleosol together with evidence from clastic dikes that this horizon was not waterlogged is an indication of an oxidizing atmosphere although probably one in which oxygen was much less abundant than it is today the very low amounts of acid titratable bases in the shaly parent material of this paleosol and surficial loss of these bases within the paleosol appear to have been produced by decarbonation reactions comparable to those found today in soils of vascular land plants a likely source of weathering acid during the precambrian is an elevated abundance of atmospheric carbon dioxide considering these likely atmospheric differences together with the distribution of comparable modern vertisols and other geological evidence the climate under which this soil formed was probably semiarid to subhumid seasonally dry and temperate

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

isotopic evidence for iron mobilization during paleoproterozoic lateritization of the hekpoort paleosol profile from gaborone botswana abstract the isotopic composition and elemental abundance of fe in a lateritic weathering profile drillcore strata 1 formed on the 2 2 ga hekpoort basalt of the pretoria group in gaborone botswana document open system behavior for fe during paleosol formation the 56 fe values of the profile increase from 0 at the bottom parental basalt to 1 in the mid depth mottled fe depleted zone then decrease to 0 3 at the top of the laterite fe enriched zone the reworked ferricrete and red beds that overlie the laterite have high 56 fe values of 0 2 to 0 4 which are comparable to those of the underlying laterite because of the absence of a complementary low 56 fe zone these results are not compatible with a model of simple internal redistribution closed system of fe within the paleosol column instead these results are well explained through fluid rock interaction involving transport of aqueous fe 2 through the system during paleosol evolution production of positive 56 fe values for fe 3 oxides were simulated through oxidation of low 56 fe fe 2 aq using experimentally determined fe 3 oxide fe 2 aq fractionation factors the increased quantities of fe 2 involved in formation of the hekpoort paleosol relative to previous estimates based on a closed system model require a commensurate increase in the abundance of an oxidant such as atmospheric o 2 to explain the fe 3 fe 2 ratios in the paleosol therefore previous estimates of atmospheric o 2 levels of 2 5 10 4 to 9 10 3 atm based on this section w yang h d holland the hekpoort paleosol profile in strata 1 at gaborone botswana soil formation during the great oxidation event american journal of science 303 2003 187 220 are too low by an order of magnitude or more these results demonstrate that fe isotopes can provide important constraints on open versus closed system behavior of fe in ancient weathering profiles a distinction that is critical when calculating fe mobility as a means for inferring surface redox conditions in the past

solute exchange by convection within estuarine sediments the rates of material transport through sediments are central to determining the biochemical status of sediments transport within sediments can occur by molecular diffusion through porewaters and by the activities of burrowing biological organisms but a number of physical processes can enhance transport within porewaters here convection is investigated as a mechanism for solute transport within sediments convection will occur as the gravitational adjustment to the occurrence of porewater which is of lower salinity and density than the water at the sediment surface this process could be important for exchange between sediments and the water column in regions subject to significant temporal variations in salinity such as estuaries

metamorphic alteration of a precambrian 2 2 ga paleosol from south africa revealed by backscattered electron imaging abstract like many precambrian paleosols the 2200 ma old waterval onder clay has been metamorphosed to the greenschist facies of regional metamorphism the extent of metamorphic alteration of the fine grained uppermost horizons of the paleosol is assessed here using optical and backscattered electron microscopy in hand specimens and under low power optical examination the paleosol shows surficial lamination cumulic horizon of soil terminology rounded clasts coated with opaque sesquioxides manganoferriargillans cracks defining a fitted breccia of blocks that were evidently soft in places blocky angular peds cracks filled with multiple generations of material from above illuviation argillans and zones of unusually highly birefringent clay climobimasepic plasmic fabric with backscattered electron microscopy however at a resolution of less than about 10 m the fine structure of originally colloidal parts of the paleosol mangans argillans matrix and plasma separations is shown to be extensively recrystallized to illite and berthierine original soil structure has been obscured at this resolution by crystal growth at random across original microscopic discontinuities preferred orientation of metamorphic crystals can still be seen along what are presumed to be highly birefringent zones of oriented clay similar to those in modern soils but the matrix itself shows random clusters of crystals suggestive of an advanced stage of illitization by ostwald ripening the style of recrystallization of laminae in the surface horizon is similar to that seen lower in the profile where recrystallized beds are thicker and coarser grained thus the paleosol probably formed on an upward thinning and fining sequence of sandstone and shale rather than on basalt on this assumption the profile has gained iron compared with its shaly parent material rather than lost iron compared with basalt this may be an indication of greater oxygenation of the atmosphere than previously thought possible microbial traces can be seen in thin section but because of pervasive recrystallization these remain dubiofossils

estimates of atmospheric o 2 in the paleoproterozoic from paleosols abstract a weathering model was developed to constrain the partial pressure of atmospheric o 2 p o 2 in the paleoproterozoic from the fe records in paleosols the model describes the fe behavior in a weathering profile by dissolution precipitation of fe bearing minerals oxidation of dissolved fe ii to fe iii by oxygen and transport of dissolved fe by water flow in steady state the model calculates the ratio of the precipitated fe iii oxyhydr oxides from the dissolved fe ii to the dissolved fe ii during weathering as a function of p o 2 an advanced kinetic expression for fe ii oxidation by o 2 was introduced into the model from the literature to calculate accurate p o 2 relationships the model s validity is supported by the consistency of the calculated p o 2 relationships with those in the literature the model can calculate p o 2 for a given paleosol once a value and values of the other parameters relevant to weathering namely ph of porewater partial pressure of carbon dioxide p co 2 water flow temperature and o 2 diffusion into soil are obtained for the paleosol the above weathering relevant parameters were scrutinized for individual paleoproterozoic paleosols the values of temperature ph and p co 2 were obtained from the literature on the paleoproterozoic paleosols the parameter value of water flow was constrained for each paleosol from the mass balance of si between water and rock phases and the relationships between water saturation ratio and hydraulic conductivity the parameter value of o 2 diffusion into soil was calculated for each paleosol based on the equation for soil o 2 concentration with the o 2 transport parameters in the literature then we conducted comprehensive p o 2 calculations for individual paleoproterozoic paleosols which reflect all uncertainties in the weathering relevant parameters consequently robust estimates of p o 2 in the paleoproterozoic were obtained 10 7 1 10 5 4 atm at 2 46 ga 10 5 0 10 2 5 atm at 2 15 ga 10 5 2 10 1 7 atm at 2 08 ga and more than 10 4 6 10 2 0 atm at 1 85 ga comparison of the present p o 2 estimates to those in the literature suggests that a drastic rise of oxygen would not have occurred at 2 4 ga supporting a slightly rapid rise of oxygen at 2 4 ga and a gradual rise of oxygen in the paleoproterozoic in long term

paleosols on the ediacaran basalts of the east european craton a unique record of paleoweathering with minimum diagenetic overprint abstract ca 548 ma old paleosols developed on ediacaran basalts and almost unaltered by diagenesis were discovered and sampled from several core profiles in volyn nw ukraine their mineral composition elemental geochemistry major and trace including ree plus fe2 fe3 ratio by mossbauer spectroscopy stable isotope geochemistry o and c in carbonates b in kaolinite and hematite age u pb in carbonates and age of diagenetic alteration k ar of illite and aluminoceladonite were studied the technique of mass balance calculation independent from bulk density measurements was applied the profiles are mineralogically and geochemically very mature primary basaltic and hydrothermal minerals disappear completely and neoformed minerals appear first dioctahedral smectite and hematite then smectite alters into kaolinite smectite and finally kaolinite and hematite remain as dominant phases geochemical indices classify volyn paleosols as paleo ultisols on the border of lateritization zone with ph close to neutral 50 60 of the rock mass was dissolved from the top layer weathering involved biological activity as documented directly by microbial mats kremer et al 2018 and supported by several geochemical indicators including the presence of immature organic matter and stable isotope ratios volyn paleosols are close analogues of recent weathering of basalts on hawaii and the coast of costa rica including the presence of mixed layer kaolinite smectite they document hot and humid ediacaran climate on the east european craton at ca 550 ma the lack of aeolian contamination implies the lack of dry regions on the craton in this period mass balance calculations suggest 90 of p loss from the top saprolitic layer given the area of the basalt trap this reflects a significant release of p into the ediacaran ocean which possibly contributed to the ediacaran life explosion and atmospheric o2 rise

chemistry and mineralogy of precambrian paleosols at the base of the dominion and pongola groups transvaal south africa abstract we have studied alteration zones found between sedimentary rocks of the lowest proterozoic dominion and archean pongola groups and underlying granitic rocks transvaal south africa the alteration zones have a transitional lower boundary primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks ca mg na p and mn are progressively lost upwards in the zone the chemical and mineralogical variations and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by precambrian weathering 2 8 3 0 ga ago after formation the paleosols underwent metasomatism and metamorphism sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays andalusite in the pre pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism the dominion paleosol was investigated at three localities the effects of weathering were similar at the three sites except for the behavior of iron one site lost about 20 of iron whereas the other sites lost 75 and 80 of initial iron content the differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography variations in iron loss may be used to place some constraints on the composition of the precambrian atmosphere given constraints imposed by dominion paleosols overlying uraniferous conglomerates requirements for chemical weathering and the greenhouse effect and possible presence of non diffusing photochemical oxidants the partial pressures of oxygen and carbon dioxide gases probably are in the ranges po1 0 02 to 0 5 pal present atmospheric level and pco2 5 to 30 pal

petrography and geochemistry of a 2000 2200 ma old hematitic paleo alteration profile on ongeluk basalt of the transvaal supergroup griqualand west south africa abstract a 4 m thick paleo alteration profile thought to represent one of the oldest known ferruginized hematitic paleoweathering zones is described from a depth of 720 m in a drill core on the farm drakenstein 263 some 20 km northwest of hotazel in griqualand west south africa the paleo alteration profile is developed on 2240 57 ma ongeluk basalt along the angular unconformity that separates transvaal strata from 2070 95 ma basal red beds of the olifantshoek group basaltic ongeluk lava was subjected to greenschist facies metamorphism represented by a mineral assemblage of fe pumpellyite chlorite albite and quartz before paleosol development the subsequent paleo alteration is characterized by concentration of fe2o3 in the form of hematite iron rich chlorite and abundant sericite the abundance of sericite results in a large excess of k2o relative to the amount of al2o3 that could have been derived from weathering of the parent basalt the origin of the excess potassium is problematical and could have been derived from basin water groundwater or metamorphic fluids field relationships favour introduction from basin water during deposition of overlying olifantshoek red beds elements such as sodium calcium and magnesium were leached from the alteration profile whereas ree became concentrated within it the most interesting aspect of the ree distribution is the preferred concentration of the lighter lanthanides la and nd in the middle part of the alteration profile relative to heavy ree and the development of a marked negative ce anomaly in the paleo weathered material hydrothermal vein alteration took place after deposition of the overlying olifantshoek red beds

a vertic paleosol at the archean proterozoic contact from the singhbhum orissa craton eastern india abstract we report on an inferred paleoproterozoic soil horizon from the singhbhum orissa craton near keonjhargarh orissa eastern india which preserves vertic vertisol like features the 2 10 m thick white shale consists of quartz muscovite pyrophyllite interpreted as a paleosol overlies 3 32 ga granite and is covered by pre 2 0 ga sandstones kolhan group deposited in marginal marine environments the paleosol passes downwards through a partly altered granite saprolith zone to fresh granite abundant pedogenic slickensides ped structures gilgai topography reddish stain on ped surfaces and sepic plasmic microfabric in the paleosol were probably related to shrink and swell processes driven by marked seasonal precipitation in a well drained landscape an observed decrease of alkali elements and an increase of silica alumina and cia values from granite to paleosol together with relatively constant ti al ratios suggest in situ weathering of granite protolith however a paleo vertisol necessitates a mafic parent rock either directly igneous or reworked through alluvium the c 2 25 ga malangtoli jagannathpur basaltic rocks are relatively widespread and close to the inferred paleosol occurrences and provide a logical solution geochemistry of the highly depleted paleosol in contrast particularly when applying the isochon method strongly supports a granitic parent the apparent conundrum can possibly be solved through repeated alluvial reworking of both granitic and mafic detritus and accumulation of weathered material with varying proportions of these two components yet enough smectitic clays 30 to form a paleo vertisol this pre 2 0 ga weathering event in the singhbhum craton is consistent with a global scenario of paleoproterozoic weathering which followed an extended global glaciation and thus has the potential to contribute to understanding of pre 2 0 ga amalgamation of regional and global cratonic blocks

behaviors of trace elements in neoarchean and paleoproterozoic paleosols implications for atmospheric oxygen evolution and continental oxidative weathering abstract the behaviors of redox sensitive and or bio essential trace elements in neoarchean and paleoproterozoic paleosols ancient weathering profiles were investigated to better understand atmospheric oxygen evolution the loss or retention of individual trace elements in the paleosols can show how continental oxidative weathering and thus atmospheric oxygen evolution took place against age mainly due to their various redox potentials the v cr ni cu zn and mo concentrations of two paleoproterozoic paleosols were measured by inductively coupled plasma optical emission spectrometry and mass spectrometry and those as well as co w and u concentrations of nine neoarchean and paleoproterozoic paleosols were obtained from the literature the trace element behaviors were constrained by their degrees of loss or retention in the paleosols we applied two methods to the estimation i retention fraction of element m a mass ratio of element m of paleosol to parent rock using immobile elements such as ti and ii element element in particular si element correlations at different profile depths of a paleosol there are two distinct groups in trace element behavior in the neoarchean and paleoproterozoic paleosols co ni zn and w were lost from weathering profiles until the early paleoproterozoic but retained in the middle and late paleoproterozoic while v cr cu mo and u were retained in the profiles until the early paleoproterozoic or slightly later but lost from the profiles in the middle and late paleoproterozoic more precisely the timings of such loss and retention were different between trace elements during the paleoproterozoic the characteristics of these changes from retention to loss or from loss to retention indicate that the changes occurred and lasted throughout the paleoproterozoic the trace element behaviors accordingly suggest that continental weathering became oxidative progressively with age during almost the whole paleoproterozoic and thus atmospheric oxygen increased similarly with age in the same time period but not suddenly in a relatively short term the loss and retention of the trace elements may infer how they were released from the continent to the ocean against age in the neoarchean and paleoproterozoic

the proterozoic thelon paleosol northwest territories canada abstract beneath the 1 72 ga thelon formation in the northwestern part of the canadian shield an unmetamorphosed and undeformed paleosol is preserved on a variety of paleoproterozoic and archean protoliths this paper documents the paleosol referred to as the thelon paleosol in four localities where it developed on leucocratic schist gneiss and granite the thelon paleosol is characterized by these ascending changes from fresh protolith through paleosol to the unconformity a saprolitic disruption of the protolith resulting in ped and ferran formation b reddening of the paleosol as a consequence of hematitization c increase in dissolution of feldspar plagioclase alkali feldspar increase in the degree of iron oxidation in biotite chlorite and garnet and an increase in the abundance of neoform kaolinite illite and hematite cement d a loss of tio2 fe2o3 total feo mno mgo cao na2o k2o p2o5 u zr rb sr ba e a gain of fe2o3 and f an increase in cia values the fabric of the parent material appears to be the only soil forming factor that locally influenced paleosol development in comparison to thin profiles developed on massive relatively impervious protoliths thick profiles have developed on metamorphic protoliths in which the fabric has allowed deeper penetration of paleoweathering fluids the behaviour of iron in the profiles particularly profile 2 with a granite protolith and an increase in hematite towards the top of the profiles suggests that the thelon paleosol formed under an oxidizing atmosphere subsequent deposition of quartzose conglomerate and sandstone of the thelon formation has caused compaction of the thelon paleosol and this compaction probably was responsible for the formation of stylocumulates in the paleosol during diagenesis of the thelon formation the influx of diagenetic fluids into the paleosol produced veinlets of quartz and hematite a newly recognized microcrystalline hydrated alumino phosphate sulphate mineral and widespread pseudomorphs of illite after pedogenic vermiform kaolinite

causes and consequences of low atmospheric pco2 in the late mesoproterozoic abstract based upon various proxy theoretical and model constraints paleoproterozoic atmospheric p co 2 was much higher than phanerozoic levels however relatively little is known about the transition between the two climate states here geochemical mass balance from 1 1 ga old midcontinent rift system usa paleosols is used to reconstruct atmospheric p co 2 during the mesoproterozoic the calculations robustly indicate low atmospheric p co 2 p co 2 minimum is best explained as the culmination of a long term c burial event by the biosphere that is also indicated by marine carbonate 13 c changes during the mesoproterozoic and which is consistent with changes in the biosphere including increased stromatolite abundance and diversity evolution of sulfur utilizing bacteria and the spread of microbial mats into continental environments

archean chemical weathering at three localities on the canadian shield abstract sapropelite developed on a granodiorite basement below the archean steeprock group in the western superior structural province of canada weathered materials from a 3 15 ga granitic basement to the archean yellowknife supergroup in the slave structural province and weathered detritus from contemporaneous komatiites and gneissic basement of the archean prince albert group in northern churchill structural province have been analysed the deviation of the weakly metamorphosed weathered material from the chemical composition of their precursors is comparable to that seen in more recent weathered profiles from similar source materials this includes the enhancement of ferric to ferrous iron ratios and potash to soda lime and magnesia ratios there is also an increase in lighter ree and a depletion of heavier ree quartz rich sediments immediately above the unconformity derived from the basement granitoids may be enriched in aluminum and iron and yield chloritoid under proper metamorphic conditions sediments formed from mixed sources such as weathered gneissic basement and contemporaneous komatiitic volcanics yield anomalous resistate sedimentary mixtures such as chromiferous quartzites based on the data given here there is no need to postulate that different weathering and or ground water systems were operative during the archean compared to more recent times on the other hand the complicated weathering and diagenetic history that altered material undergoes below a subsiding unconformity relies as much or more on the buffering capacity of the enclosed rock mass as on the atmospheric conditions prevalent at the time of weathering hence weathering profiles old and new are more a response to local environmental conditions within the rock than the composition of the atmosphere

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

profiles of elemental concentrations in precambrian paleosols on basaltic and granitic parent materials abstract profiles of elemental concentrations are interpreted for four precambrian paleosols two developed on basalt and two on granodiorite all four paleosols appear to be the erosional remnants of originally thick soil saprolite regoliths the granitic paleosols are in south africa where they underlie the 2 9 3 0 ga pongola supergroup and the 2 8 2 9 ga dominion reef conglomerate one of the basaltic paleosols also occurs in south africa where it caps the ventersdorp basalt and underlies the 2 3 ga black reef quartzite the other basaltic paleosol underlies sandstone of the 2 7 ga timiskaming group in the abitibi belt of the canadian shield all four paleosols exhibit pronounced upward loss of sodium but no consistent loss of the heavier alkali elements rubidium and cesium iron decreases upward above iron rich basaltic parent rocks but there is no consistent loss of iron above iron poor granitic parents the rare earth elements are less fractionated than during intensive modern weathering uranium locally has been fractionated from thorium possibly due to oxidative dissolution during precambrian weathering

effects of current bed form induced fluid flow on the thermal regime of sediments 1 heat transport associated with current bed form fluid exchange affects biogeochemical and ecological process in a variety of aquatic settings we simulate turbulent flow over dunes induced darcy flow in sediments and sediment heat transport forced by a diel variation of water column temperature heat conduction dominates when the water column current cannot induce rapid fluid exchange heat advection becomes important with increasing sediment permeability and or current velocity resulting in a complicated space time temperature pattern weak temperature variations are found beneath dune crests where water upwells out of the sediments strong variations are located below dune stoss faces where water downwells the relative dominance of heat conduction versus advection is captured by a global peclet number that depends on dune geometry and hydrodynamic and thermal properties including the period of thermal forcing thermally induced fluid viscosity variations cause periodic but nonsymmetric changes in fluid flux through the sediments but do not significantly influence temperature patterns

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

nd and sr isotopic characteristics of chinese deserts implications for the provenances of asian dust silicate nd sr isotopes of the fine grained fractions of the 10 major deserts and sandy lands in north china and the loess in chinese loess plateau were systematically investigated wide ranges in nd sr isotopic compositions have been observed the results of the 7 0 b the deserts on the northern margin of tibetan plateau with end 0 ranging from 11 9 to 7 4 and c the deserts on the ordos plateau with the lowest end 0 11 5 the distribution of the threes isotopic regions is controlled by the tectonic setting in north china which implies that the materials of the deserts are derived from the locally eroded rocks from the surrounding mountains and the nd sr isotopic signatures of these deserts could be quit stable over the past million years on the sub tectonic time scales if there is any desert at those times the nd sr isotopic compositions of the loess are mostly close to those of the deserts in isotopic region b suggesting that the main source regions of the last glacial loess in the chinese loess plateau are badain jaran desert tengger desert and qaidam desert also the comparison between the nd sr isotopes of the 5 lm silicate fractions of the deserts and the ancient dust falls in the north pacific and greenland show that the asian end members of these dust falls are derived most from the deserts in the isotopic region b and less from those in the isotopic region c

pb o and c isotopes in silicified mooidraai dolomite transvaal supergroup south africa implications for the composition of paleoproterozoic seawater and dating the increase of oxygen in the precambrian atmosphere abstract the mooidraai dolomite formation is a unit of marine sedimentary carbonates in the upper transvaal supergroup which recorded significant changes in the composition of the earth s atmosphere and ocean previously available data had suggested that this dolomite was about 2 2 ga old and showed 13ccarb values around 0 8 pdb and hence was an exception to the positive excursion of 13ccarb values observed worldwide in marine carbonates deposited between 2 25 and 2 05 ga ago we studied the pb o c isotope systematics of drill core samples from a highly silicified and aluminosilicate free sub unit of the mooidraai dolomite formation that yields well preserved micritic dolomite grains selective leaching of the silicified dolomite revealed a significant pb isotopic contrast between the carbonate and the quartz fraction the former shows only a narrow range of 208pb 204pb 35 35 35 58 which does not correlate with 206pb 204pb indicating the absence of th derived detrital pb the carbonate bound pb defines an isochron that corresponds to an age of 2394 26 ma 2 mswd 1 8 n 22 which we interpret as the diagenetic age of the mooidraai dolomite the 13ccarb values range from 0 51 to 0 64 pdb and confirm the previous data the 18ocarb values range from 2 08 to 0 18 pdb and are amongst the heaviest reported yet from early precambrian marine sedimentary carbonates the quartz fraction is considerably lighter than the carbonate fraction and shows 18oqtz values between 9 22 and 6 73 pdb 21 40 and 23 97 smow together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 c this indicates that the intense silicification was post depositional comparison of 18ocarb values of silicified and non silicified mooidraai samples suggests that the post depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite the heavy 18ocarb values suggest that 2 4 ga ago the oxygen isotopic composition of seawater differed only little from that of today the new pb pb carbonate age for the mooidraai dolomite indicates that the upper transvaal supergroup is about 200 my older than previously thought this explains the normal carbon isotopic composition of the mooidraai dolomite and suggests that the pronounced increase of the oxygen content in the precambrian earth s atmosphere that is seen in strata underneath the mooidraai dolomite occurred before 2 4 ga ago on this background it appears unlikely that the positive excursion of the carbon isotopic ratios of 2 25 2 05 ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen

sr and nd isotopic composition of dust in dunde ice core northern china implications for source tracing and use as an analogue of long range transported asian dust abstract this study presents the sr and nd isotopic composition of dust from a shallow dunde ice core 38 06 n 96 24 e 5325 m a s l northern china in order to trace its source regions and the provenance of long range transported lrt asian dust dunde dust samples show similar sr 87 sr 86 sr averages 0 719982 and varies between 0 715220 and 0 721874 and nd e nd 0 averages 10 6 and ranges from 11 1 to 9 9 isotopic composition to desert sand from taklimakan and qaidam revealing that the tarim and qaidam basins are the most possible source areas for dunde dust while the contribution from badain jaran and tengger seems very small or unlikely our results indicate that materials from taklimakan can be entrained from the tarim basin and transported to qilian shan by westerlies under the present climate the sr nd isotopic composition of dunde dust which is a good analogue for the long range transported asian dust resembles that of north pacific and greenland dust indicating that the important provenance of end members of the asian dust seems to be taklimakan on a regional scale

pore water chemistry in mangrove sediments relationship with species composition and developmental stages french guiana spatial and seasonal variability of sedimentary salinity ph redox potential and solid phase sulphide concentration were investigated in a range of mangrove communities along the coast of french guiana seasonal depth distributions of these parameters and organic content were compared within avicennia rhizophora and mixed mangrove stands at different stages of plant development mangrove communities and variable surface water inputs strongly impact sediment and ground water properties in the upper sediment changes in salinity are mainly controlled by seasonal conditions transpiration and proximity of fresh water influx whereas we suggest that constant basal salinity results from an accumulation of salt that has migrated as a result of density driven convection processes there are no clear differences between the depth distributions of salinity obtained beneath avicennia germinans and rhizophora mangle stands implying that plant zonations are not primarily controlled by soil salinity in this environment nevertheless r mangle grows in places subjected to the greatest variability in freshwater influxes suggesting that rhizophora might require or withstand occasional inundation by fresh water beneath rhizophora stands sediment properties reflect anaerobic and sulphidic conditions close to the sediment surface in contrast beneath avicennia stands sediment geochemistry mostly depends on the stages in forest development on contents in sedimentary organic matter and on seasonal changes in the early stage of avicennia settlement the sediment at the level of radial pneumatophore bearing cable roots displays permanent suboxic conditions with eh values reaching 400 mv these high ehs are interpreted as an effect of the oxidation produced by the cable root system the development of mature avicennia stands results in accumulation of sedimentary organic matter and promotes low ehs and the reduction of pore water sulphate near cable root level the oxidation process observed in pioneer mangroves results in a reoxidation of solid sulphides produced previously during dry conditions the desiccation of the upper sediment adds its oxidation effects to those of root activity as a result suboxic processes dominate in the upper 20 cm thick layer organic matter decomposition and sulphur oxidation strongly acidify the sediment below 20 cm the sediment is anaerobic and sulphidic hence sulphide concentrations depend on the edaphic conditions controlling decay processes and appear to be a consequence rather than a cause of the observed zonation of vegetal species the small size of a germinans propagules might have a significant influence on the extensive development of this plant community along the highly dynamic coastline of the guianas this study demonstrates that the different properties of pore water were intimately linked and that the explanation of the evolution of this forest reflects a combination of multiple parameters moreover it appeared that the organic content played a key role along with the species composition and the seasonal variations waterlogging desiccation

physicochemical environments for the formation of quartz free manganese oxide ores from the early proterozoic hotazel formation kalahari manganese field south africa physicochemical environments for the formation of quartz free manganese oxide ores from the hotazel formation of the kalahari manganese field northern cape province south africa are discussed quantitatively on the basis of the phase relations in the system mn fe si o and qualitatively in the system ca mn fe si o the phase relations involve major oxide minerals of the ores namely braunite braunite ii hausmannite bixbyite jacobsite and hematite and were constructed under quartz deficient conditions using existing and estimated thermody namic data and mineral assemblages from the ores standard enthalpy delta hm 0 sub f 298 and gibbs free energy delta g 0 sub f 298 values of braunite are estimated to be 1 011 9 or 5 and 937 3 or 5 kcal mole respectively on the basis of published experimental data and predicted entropy and heat capacity the low grade diagenetic to very low grade metamorphic mamatwan type ores consisting mainly of braunite hausmannite jacobsite hematite kutnohorite and calcite are altered by hydrothermal solutions into the high grade high temperature wessels type ores consisting of braunite ii bixbyite hausmannite hematite calcite and andradite with trace amounts of braunite the peak temperature of formation of the high grade ores appears not to have exceeded about 300 degrees to 400 degrees c the ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite jacobsite hematite to that defined by the assemblage bixbyite hausmannite with increasing temperature under hydrothermal quartz deficient conditions the stability of braunite is enlarged to higher temperatures with increasing silica activity a sub sio sub 2 aq because braunite is much less abundant in the high grade wessels type ore than in the low grade mamatwan type ore the maximum a sub sio sub 2 aq in the solutions could be estimated at 10 super 4 to 10 super 3 at 300 degrees to 400 degrees c from the stable assemblage bixbyite hausmannite braunite kutnohorite and to some extent calcite was a major source of ca reacting with silica released from braunite to form braunite ii and andradite

paleomagnetism of a lateritic paleoweathering horizon and overlying paleoproterozoic red beds from south africa implications for the kaapvaal apparent polar wander path and a confirmation of atmospheric oxygen enrichment the olifantshoek group in southern africa contains paleoproterozoic red beds that are exceptionally well preserved lying unconformably atop a regionally extensive lateritic paleoweathering profile we studied the basal unit of this succession known as the gamagara or mapedi formation and the lateritized substrate so called drakenstein or wolhaarkop paleosol on which it developed two ancient magnetic components are observed one int usually with a distributed unblocking spectrum between 300 and 600 c but occasionally persisting to 675 c is directed shallowly southward or northward a mesoscale fold test at south sishen mine indicates that this component was acquired during deformation similarity of the direction to previous results suggests that it was acquired at 1240 ma during early namaqua orogenesis combining our int results with existing data from the namaqua eastern zone nez we calculate the nez pole at 44 9 n 021 5 e k 23 2 a 95 12 8 q 5 the most stable component from our data set hig always persisting as a nonzero endpoint to demagnetization at 665 680 c is observed in 32 samples from south sishen and beeshoek mines directed moderately east downward sishen or west upward beeshoek this component predates the mesoscale fold at sishen more importantly a conglomerate test at beeshoek indicates that hig was acquired prior to paleoproterozoic deposition of the gamagara mapedi formation the concordance between directions from the paleoweathering zone and immediately overlying red beds indicates that hig is a primary magnetic remanence for the basal gamagara mapedi bgm formation the dual polarity bgm paleomagnetic pole 02 2 n 081 9 e dp 7 2 dm 11 5 q 6 lies neatly between previous kaapvaal poles with ages of 2220 ongeluk lavas and 2060 ma bushveld complex our data thus support recent correlations of the gamagara mapedi formation with pre bushveld sediments of the pretoria group a pre bushveld age for bgm is also consistent with its substantial distance from a new albeit less reliable paleomagnetic pole from the 1930 ma hartley lavas higher within the olifantshoek succession 12 5 n 332 8 e k 18 6 a95 16 0 q 3 our conglomerate test at beeshoek confirms previous allegations that the intense hematitization observed in the drakenstein wolhaarkop paleosol occurred during paleoproterozoic weathering under a highly oxygenated atmosphere

the flin flon paleosol revisited a ca 1 85 ga paleosol developed on seafloor altered mafic rocks is preserved beneath continental sandstones and conglomerates of the missi group near flin flon manitoba previous work has focused

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

the driving forces of porewater and groundwater flow in permeable coastal sediments a review abstract advective flows rapidly transport water solutes and particles into and out of permeable sand beds and significantly affects the biogeochemistry of coastal environments in this paper we reviewed the drivers of porewater and groundwater advection in permeable shelf sediments in an attempt to bridge gaps among different disciplines studying similar problems we identified the following driving forces 1 terrestrial hydraulic gradients 2 seasonal changes in the aquifer level on land moving the location of the subterranean estuary 3 wave setup and tidal pumping 4 water level differences across permeable barriers 5 flow and topography induced pressure gradients 6 wave pumping 7 ripple and other bed form migration 8 fluid shear 9 density driven convection 10 bioirrigation and bioturbation 11 gas bubble upwelling and 12 sediment compaction while these drivers occur over spatial scales ranging from mm to km and temporal scales ranging from seconds to years their ultimate biogeochemical implications are very similar i e they are often a source of new or recycled nutrients to seawater and transform organic carbon into inorganic carbon drivers 2 12 result in no net water input into the ocean taking all these mechanisms into account we conservatively estimate that a volume equivalent to that of the entire ocean is filtered by permeable sediments at time scales of about 3000 years quantifying the relative contribution of these drivers is essential to understand the contribution of sediments to the global cycles of matter