permian and triassic greenhouse crises paleoclimatic time series from permian and triassic paleosols reveal transient episodes of unusually warm and wet conditions interrupting long periods of cool and dry conditions usual for calcareous red paleosols some of these paleoclimatic events are known from stomatal index of fossil lepidopteris leaves to have been episodes of elevated global atmospheric co2 the magnitude of 19 known permian and triassic greenhouse crises varied considerably and they offer new evidence for the relationship between paleoclimate and atmospheric co2 levels these greenhouse crises also had marked effects on global lowland vegetation introducing frost sensitive tropical lycopsids to high latitudes and drought tolerant conifers to low latitude lowlands greenhouse events punctuate phases in plant evolution ottokaria callipteris plumsteadia rufloria lidgettonia tatarina pleuromeia and dicroidium scytophyllum floras greenhouse events also punctuate the evolution of reptilian dynasties successive pelycosaur dinocephalian dicynodont rhynchosaur and dinosaur faunas and respiratory adaptations such as enlarged bony secondary palate greenhouse crises of the late and middle permian were the most severe known and suggest a role for atmospheric pollution with ch4 and co2 in those mass extinction events probably from thermogenic cracking of coals by intrusive feeder dikes of flood basalts because of formalities in boundary definition these mass extinctions are neither end permian nor end guadalupian but upper changhsingian and mid capitanian respectively

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

palaeoclimatology archaean palaeosols and archaean air arising from h ohmoto y watanabe k kumazawa nature 429 395 399 2004 see also communication from j f kasting h ohmoto y watanabe reply ferrous carbonate as the mineral siderite occurs in archaean palaeosols ancient soils ohmoto et al 1 contend that siderite was not in equilibrium with the oxygen in archaean air and that its presence in palaeosols provides little constraint on the partial pressure of carbon dioxide in archaean air but their argument is invalid because it fails to distinguish the different behaviours of the trivial component oxygen and the significant component carbon dioxide in the partly closed system of soil waters the presence or absence of siderite in ancient soils is a valid constraint on the carbon dioxide partial pressure pco2 in ancient atmospheres

a bistable organic rich atmosphere on the neoarchaean earth it has been hypothesized that before widespread oxygenation about 2 45 billion years ago the earth s atmosphere contained an organic haze similar to that on titan however these theoretical predictions have not been substantiated by geological evidence here we use multiproxy geochemical analyses of sediments from the 2 65 2 5 billion year old ghaap group in south africa to reconstruct ocean and atmospheric chemistry during this time we find evidence for oxygen production in microbial mats and localized oxygenation of surface waters carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane consistent with the prediction of a hydrocarbon haze we use a photochemical model to corroborate our geochemical data our simulations predict transitions between two stable atmospheric states one with organic haze and the other haze free the transitions are presumably governed by variations in the amount of biological methane production during the archaean eon we find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the archaean before the rise of oxygen the atmosphere of the early earth may have consisted of an organic haze geochemical data and modelling suggest that from 2 65 to 2 5 gyr ago several transitions between hazy and haze free atmospheric conditions occurred potentially linked to variations in biogenic methane production

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 flin flon paleosol and the composition of the atmosphere 1 8 bybp within the 1800 to 1900 my old flin flon snow lake greenstone belt amisk group volcanics are overlain by missi group fluvial sediments several localities along the missi amisk contact the volcanics show evidence of subaerial weathering field relationships mineralogical evidence and chemical analyses confirm that this alteration zone is a paleosol pedogenic fabrics and mineralogy were somewhat obscured by greenschist grade metamorphism associated with the hudsonian orogeny 1750 my this is especially true in the upper meter of the paleosol where metamorphic paragonite and sericitic micas developed in a crenulated fabric this metamorphism did not however obliterate the imprint of weathering on the amisk volcanics features characteristic of well drained modern soils are evident in the paleosol corestones of spheroidally weathered pillow lavas occur at depth within the paleosol cr horizon the corestones decrease in size upward and eventually disappear into a hematite rich horizon at the top of the paleosol these macroscopic changes are accompanied by a decrease in cao and mgo and by an increase in al2o3 tio2 and total iron toward the paleosol missi contact ferrous iron decreases upward toward the contact feo was apparently oxidized to ferric iron and retained within the paleosol during weathering the oxidation and retention of iron within the flin flon paleosol indicates that po2 was probably or 10 2 p a l at the time of weathering the behavior of iron in the flin flon paleosol contrasts sharply with its behavior in the 2200 my hekpoort paleosol which is strongly depleted in iron this difference suggests that a significant increase in the ratio of po2 pco2 in the atmosphere took place between 2200 and 1800 mybp

silicon isotope fractionation during microbial reduction of fe iii si gels under archean seawater conditions and implications for iron formation genesis abstract microbial dissimilatory iron reduction dir is a deeply rooted metabolism in the bacteria and archaea in the archean and proterozoic the most likely electron acceptor for dir in marine environments was fe iii si gels it has been recently suggested that the fe and si cycles were coupled through sorption of aqueous si to iron oxides hydroxides and through release of si during dir evidence for the close association of the fe and si cycles comes from banded iron formations bifs which consist of alternating bands of fe bearing minerals and quartz chert although there has been extensive study of the stable fe isotope fractionations produced by dir of fe iii si gels as well as studies of stable fe isotope fractionations in analogous abiologic systems no studies to date have investigated stable si isotope fractionations produced by dir in this study the stable si isotope fractionations produced by microbial reduction of fe iii si gels were investigated in simulated artificial archean seawater aas using the marine iron reducing bacterium desulfuromonas acetoxidans microbial reduction produced very large 30 si 28 si isotope fractionations between the solid and aqueous phase at 23 c where 30 si solid aqueous isotope fractionations of 3 35 0 16 and 3 46 0 09 were produced in two replicate experiments at 32 fe iii reduction solid phase fe ii fe total 0 32 this isotopic fractionation was substantially greater than that observed in two abiologic controls that had solid phase fe ii fe total 0 02 0 03 which produced 30 si solid aqueous isotope fractionations of 2 83 0 24 and 2 65 0 28 in a companion study the equilibrium 30 si solid aqueous isotope fractionation was determined to be 2 3 for solid phase fe ii fe total 0 collectively these results highlight the importance of fe ii in fe si gels in producing large changes in si isotope fractionations these results suggest that dir should produce highly negative 30 si values in quartz that is the product of diagenetic reactions associated with fe si gels such si isotope compositions would be expected to be associated with fe bearing minerals that contain fe ii indicative of reduction such as magnetite support for this model comes from recent in situ si isotope studies of oxide facies bifs where quartz in magnetite rich samples have significantly more negative 30 si values than quartz in hematite rich samples

the 1100ma sturgeon falls paleosol revisited implications for mesoproterozoic weathering environments and atmospheric co2 levels abstract during the active rifting stage of the 1100 ma midcontinental rift in north america alluvial sediments were deposited intermittently between basalt flows on the north and south shores of present day lake superior at times of depositional quiescence paleosols developed in both areas on the alluvial sediments and on the antecedent basalt new results from the sturgeon falls paleosol in michigan characterizing the weathering processes at the time of its formation indicate moderate maturity high degrees of hydrolysis and leaching and a low degree of salinization geochemical provenance indices indicate a homogeneous source for the paleosols and in contrast to earlier work there is little evidence for k metasomatism as a result atmospheric co 2 levels of 4 6 pre industrial atmospheric levels were calculated using a mass balance model this result is consistent with previous calculations from nearly contemporaneous paleosols from the other side of the keweenawan rift and from the 100 ma younger sheigra paleosol in scotland the calculated co 2 values are also consistent with the calculated weathering environment proxies that indicate weak to moderate weathering at this time frame and suggest that the higher greenhouse gas loads indicated by paleoproterozoic paleosols had dissipated by the mid late mesoproterozoic

the calcium carbonate saturation state in cyanobacterial mats throughout earth s history through early lithification cyanobacterial mats produced vast amounts of caco3 on precambrian carbonate platforms before 540 myr ago the superposition of lithified cyanobacterial mats forms internally laminated macroscopic structures known as stromatolites similar structures can be important constituents of phanerozoic carbonate platforms 540 myr to present early lithification in modern marine cyanobacterial mats is thought to be driven by a metabolically induced increase of the caco3 saturation state xcaco3 in the mat however it is uncertain which microbial processes produce the xcaco3 increase and to which extent similar xcaco3 shifts were possible in precambrian oceans whose chemistry differed from that of the modern ocean i developed a numerical model that calculates xcaco3 in cyanobacterial mats and used it to tackle these questions the model is first applied to simulate xcaco3 in modern calcifying cyanobacterial mats forming at highborne cay bahamas it shows that while cyanobacterial photosynthesis increases xcaco3 considerably sulphate reduction has a small and opposite effect on mat xcaco3 because it is coupled to h2s oxidation with o2 which produces acidity numerical experiments show that the magnitude of the xcaco3 increase is proportional to dic in dic limited waters dic 3 10 mm is proportional to ph when ambient water dic is not limiting and always proportional to the concentration of ca2 in ambient waters with oceanic ca2 concentrations greater than a few millimolar an appreciable increase in xcaco3 occurs in mats under a wide range of environmental conditions including those supposed to exist in the oceans of the past 2 8 gyr the likely lithological expression is the formation of the microsparitic stromatolite microtexture indicative of caco3 precipitation within the mats under the control of microbial activity which is found in carbonate rocks spanning from the precambrian to recent the model highlights the potential for an increase in the magnitude of the xcaco3 shift in cyanobacterial mats throughout earth s history produced by a decrease in salinity and temperature of the ocean a decrease in atmospheric pco2 and an increase in solar irradiance such a trend would explain how the formation of the microsparitic stromatolite microtexture was possible as the xcaco3 of the ocean decreased from the paleoproterozoic to the phanerozoic

well preserved late precambrian paleosols from northwest scotland abstract two paleosol profiles were developed side by side on gneiss sheigra clay paleosol and amphibolite staca clay paleosol along the same unconformable contact of lewisian metamorphic rocks 2 7 1 7 ga overlain by torridonian sandstones 0 8 ga in northwestern scotland although altered during burial by compaction and illitization these profiles are well preserved compared with most other precambrian paleosols that are metamorphosed to at least greenschist facies in addition deformation of pegmatite veins in a manner similar to soil creep can be taken as an indication that upper horizons of these paleosols have been preserved soil development in both profiles was characterized by the oxidation of iron in biotite and magnetite and by hydrolysis of plagioclase orthoclase and hornblende clay skins soil like microfabrics and pedogenic smectite have been preserved in the profiles alteration of silicate grains other than quartz microcline and biotite has been intense but chemical analyses failed to show dramatic removal of alkalis and alkaline earths from the profiles microscopic concretions similar to pedogenic carbonate were found in subsurface horizons of the staca clay rare earth elements are more abundant within the paleosols than in their parent materials these paleosols developed on a well drained landscape that was buried by torridonian alluvial fans atmospheric oxidation was greater than revealed by studies of archean paleosols but probably still short of modern levels paleoclimate was subhumid and probably seasonal organic carbon and its light isotopic composition in the paleosol is evidence for photosynthetic microbial crusts paleosols of such great antiquity do not fit comfortably within modern soil taxonomies and are for the moment assigned to the informal category of green clays

anoxic dissolution processes of biotite implications for fe behavior during archean weathering abstract iron rich biotite fe mg 5 dissolution experiments were carried out in a batch system under o 2 deficient conditions p o 2 5 atm referred to as anoxic conditions at 1 atm of p co 2 ph 4 6 and 100 c for 7 120 days for a better understanding of anoxic weathering processes and fe behavior during weathering before 2 2 ga for comparison oxic fe rich biotite dissolution experiments were conducted under present atmospheric conditions at ph 4 7 and 100 c for 7 80 days referred to as oxic conditions by using the same starting biotite as that for the anoxic experiments the concentrations of fe in solution after the dissolution experiments were larger by one to more than two orders of magnitude under anoxic conditions than under oxic conditions high resolution scanning and transmission electron microscopy revealed that fe ii rich vermiculite or smectite was precipitated as a secondary mineral at the edge of biotite under anoxic conditions in contrast to the formation of fe iii and al hydr oxides under oxic conditions the results suggest that part of fe ii is released to water as anoxic weathering proceeds explaining the decrease of fe content in pre 2 2 ga paleosols relative to their parent rocks the fe mg molar ratio of the secondary vermiculite or smectite was more than 7 while the starting fe rich biotite had a value of about 5 the fe mg molar ratio was less than 2 5 in solution the results of the anoxic experiments suggest that fe ii rich vermiculite or smectite could be the precursor of the chlorite preserved in pre 2 2 ga paleosols this is further corroborated by the increase in fe mg molar ratios in chlorite with decreasing depth in some precambrian paleosols

life associated with a 2 76 ga ephemeral pond evidence from mount roe 2 paleosol dark sericitic material at and near the top of the 2 765 0 01 ga mount roe 2 paleosol in western australia contains 0 05 0 10 wt organic carbon with delta 13c values between 33 and 51 pdb peedee belemnite such negative isotopic values strongly indicate that methanotrophs once inhabited this material the textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds that these ponds became desiccated and that heavy rains transported the material to its present sites the discovery of methanotrophs associated with the mount roe 2 paleosol may extend their geologic record on land by at least 1 5 b y methanotrophy in this setting is consistent with the notion that atmospheric methane levels were or 20 mu atm during the late archean the radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the late archean

low oxygen levels in earliest triassic soils an earliest triassic methane postapocalyptic greenhouse following the permian triassic p t extinction event was proposed on the basis of evidence of deeply weathered paleosols at high latitudes with features of low latitude soils and low stomatal index values of seed fern leaves reexamination of distinctive phyllosilicates unique to a single stratigraphic level in paleosols located just above the isotopically defined permian triassic boundary in australia and antarctica furnishes additional tests of this hypothesis illite is the dominant clay mineral in earliest triassic paleosols from antarctica but the paleosols also contain conspicuous green nodules of coarsely crystalline berthierine examples from the geologic record and from experimental studies indicate that the formation of berthierine is restricted to reducing conditions the occurrence of this unusual mineral in soils may indicate soil oxygen consumption by the influx of atmospheric methane to form carbon dioxide which in turn warmed the earliest triassic giving rise to a postapocalyptic greenhouse

paleoproterozoic ca 1 9 ga megascopic life on land in western australia abstract controversial hairpin shaped trace fossils myxomitodes stirlingensis and discoid fossils cyclomedusa davidi are here reinterpreted in a reassessment of sedimentology and paleosols of the 1 9 0 1 ga stirling range formation of western australia paleosols in the stirling range formation were recognized from complex cracking patterns peds and cutans chemical and mineralogical zonation soil horizons and poikiloblastic crystals desert roses redox profiles of the paleosols are evidence of well drained profiles of a coastal plain under a low oxygen 433 116 ppmv and carbon dioxide 564 64 ppmv atmosphere gypsum pseudomorphs in paleosols that are deeply weathered chemically represent a persistence of acid sulfate weathering better known in archean paleosols before the great oxidation event at 2 45 ga chemical composition of the paleosols is evidence of a humid 1478 182 mm mean annual precipitation temperate 11 3 0 5 c mean annual temperature paleoclimate their paleoenvironmental setting was a coastal plain on a passive tectonic margin megafossils were found on paleosols of western red sandstones of the stirling range formation not in gray shales and sandstones of marine facies to the east trace fossils myxomitodes stirlingensis and discoid fossils are both found at the surface of well drained paleosols and so were not metazoan trails nor sea jellies myxomitodes was more likely a grex slug trail of soil slime molds and the cyclomedusa discoids were probably microbial colonies pervasive microfilamentous structures in the paleosol surface are additional evidence of paleoproterozoic microbial earth ecosystems

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

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

metasaprolite in the mcgrath gneiss minnesota usa viewing paleoproterozoic weathering through a veil of metamorphism and metasomatism abstract a 2100 ma 420 cm thick metasaprolite which is devoid of plagioclase but contains abundant microcline occurs in the archean mcgrath gneiss beneath the overlying paleoproterozoic denham formation despite being recrystallized under amphibolite facies conditions and chemically modified by potassium metasomatism mass fluxes related to weathering and metasomatism of the mcgrath metasaprolite can be estimated by judicious application of several geochemical parameters and the a c n k plot fedo et al 1995 the metasaprolite yields values of 67 94 for the plagioclase index of alteration 62 72 for the chemical index of alteration and 47 for the feldspar index of weathering and the total mass flux removed by weathering is a minimum of 2 4 moles cm 2 such values correspond to an intermediate degree or intensity of weathering compared to other more intensely weathered paleosols in the region such as those beneath the baraboo and sioux quartzites potassium metasomatism occurred at 1742 ma during the geon 17 yavapai tectonothermal event when a minimum of 0 33 moles cm 2 k 2 o was added to the metasaprolite although the absence of pedogenic textures precludes interpretation of climatic conditions during weathering of the mcgrath gneiss the level of atmospheric p co 2 can be estimated by application of sheldon s 2006 method which yields a minimum of 3 7 pal for a weathering duration of 100 000 years

chemical and textural overprinting of ancient stromatolites timing processes and implications for their use as paleoenvironmental proxies abstract shale normalized rare earth element ree sn patterns and transition metal abundances in banded iron formations bif and shales have long been used as paleoenvironmental proxies to investigate the evolving geochemistry of precambrian oceans apparent similarities between the ree sn of modern stromatolites and contemporary seawater values also mean that some ancient stromatolites could record paleo ocean chemistry to test this hypothesis we present comparative mineralogical textural and spectral analyses of multifurcate silicified and iron oxide carbonate rich stromatolites from the ca 1 88 ga gunflint formation ontario canada previous studies show that these organo sedimentary structures represent the initiation of shallow marine sedimentation in the animikie basin on the southern tip of the superior province we present evidence that these structures were originally stabilized by aragonite and mg bearing carbonates fabrics indicative of stromatolite accretion under local hypersaline conditions are also present with la anomalies suggestive of laterally variable freshwater runoff the admixture of partially evaporated seawater and silica oversaturated deep seawater led to pervasive silicification of metastable carbonates and preservation of less soluble early diagenetic carbonate cements bulk rock transition metal contents point to lateral variability and dissimilar partitioning and fractionation during mineral formation yet ce anomalies amongst samples are similar considering the pronounced differences between the ree partitioning between fe oxyhydroxide precipitates and carbonates our results point to hematite and siderite as secondary minerals that postdate the stabilization of these structures by carbonates and silica stratabound relations of the fe rich stromatolites with diabase sills and dikes indicate that the pore water fluids evolved towards a localized state of hematite and siderite oversaturation towards the mesoproterozoic if hematite and siderite are both early diagenetic phases then the ce anomalies of these stromatolites may be a poor measure of the redox conditions of the shallow areas of the animikie basin at the time of accretion

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

pedogenesis and geochemical alteration of the picture gorge subgroup columbia river basalt oregon reddened clay rich horizons between basalt flows have historically been identified as baked zones or as zones of lateral groundwater movement the reddening has been attributed to thermal or chemical alteration of the permeable portion of the basalt flow many of these red horizons are instead paleosols that record significant hiatuses in basalt flow emplacement paleosols were identified in the field on the basis of features such as horizonation ped morphology and root traces terrestrial gastropods fossil plants and peats also occur locally plots of weathering indices such as base loss are consistent with a pedogenic origin for these horizons ca na mg fe 21 and p have been extensively lost from the a and bt horizons of the paleosols consistent with modern weathering patterns k and rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles relative to the parent material with rb added disproportionately historically this pattern of enrichment has been cited as evidence of k metasomatism the apparent metasomatism can be explained using a new model in which there was extensive felsic volcanism contemporaneous with pedogenesis and k and rb were likely added by windborne ash the apparent discordance between k and rb contents of the paleosols can be explained by the presence of land plants because k is an important electrolyte and rb does not serve any major biological role this model could account for apparent k metasomatism in other paleosols following the advent of land plants and may be applicable to older paleosols as well

isotope composition and volume of earth s early oceans oxygen and hydrogen isotope compositions of earth s seawater are controlled by volatile fluxes among mantle lithospheric oceanic and continental crust and atmospheric reservoirs throughout geologic time the oxygen mass budget was likely conserved within these earth system reservoirs but hydrogen s was not as it can escape to space isotopic properties of serpentine from the approximately 3 8 ga isua supracrustal belt in west greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater archaean oceans were depleted in deuterium expressed as d relative to vienna standard mean ocean water vsmow by at most 25 5 but oxygen isotope ratios were comparable to modern oceans mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in earth s oceans our calculations predict that the oceans of early earth were up to 26 more voluminous and atmospheric ch4 and co2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on archaean earth

greenhouse crises of the past 300 million years proxies of past co 2 and climate over the past 300 m y now reveal multiple global climate change events in unprecedented detail evidence for past co 2 spikes comes from expanded and refined data on stomatal index of fossil ginkgo and related leaves new evidence for synchronous climatic change comes from paleosols in montana utah and neighboring states each co 2 spike was coeval with unusually clayey red and decalcified paleosols which can be traced throughout the colorado plateau spikes in atmospheric co 2 also were coeval with increases in paleosol alkali depletion as an indication of high temperature and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation in the colorado plateau times of warmer climate were also more humid perhaps due to greater moisture potential of warmer air seasonality of climate did not increase during warm wet spikes the mesozoic greenhouse was not persistently hot with cool spells but warm with hot flashes these data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature mat and mean annual precipitation map due to rising co 2 in a mid latitude mid continental region the magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions

co2 drawdown and cooling at the onset of the great oxidation event recorded in 2 45 ga paleoweathering crust abstract the archean atmosphere is thought to have been devoid of oxygen but instead containing high concentrations of greenhouse gases such as co2 and possibly ch4 that were required to keep the earth surface environments warm enough for maintenance of liquid water under faint young sun conditions earlier studies have suggested that the co2 concentrations must have been as high as over 1000 times present atmospheric levels pal to overcome the effects of weaker solar luminosity however more recent studies of precambrian paleosols imply existence of at least tenfold lower but still high pco2 levels the appearance of minute amounts of atmospheric o2 at the transition of archean and proterozoic is well documented however the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated different paleoclimate proxies of the 2 45 ga kuksha paleoweathering crust suggest its formation in a cool temperate climate paleoatmospheric pco2 estimates by geochemical and isotopic mass balance models suggest remarkably low paleoatmospheric co2 levels with the best guess estimates of atmospheric co2 between 1 and 10 pal we speculate that the kuksha paleoweathering crust either predates or partly overlaps with the onset of huronian glaciation and corresponds to a time interval of low co2 level during the first recorded large glaciation in earth history

revisiting hcn formation in earth s early atmosphere abstract using a new photochemical model the hcn chemistry in earth s early atmosphere is revisited we find that hcn production in a ch 4 rich early atmosphere could have been efficient similar to the results of a previous study zahnle 1986 for an assumed ch 4 mixing ratio of 1000 ppmv hcn surface deposition increases from 2 10 9 cm 2 s 1 at fco 2 3 to more than 1 10 10 cm 2 s 1 30 tg yr at fco 2 0 3 and 1 these conditions may well have applied throughout much of the archean eon 3 8 2 5 ga prior to the origin of life and the advent of methanogens hcn production rates would likely have been at 1 10 7 cm 2 s 1 or lower thereby providing a modest source of hcn for prebiotic synthesis

low hesperian pco2 constrained from in situ mineralogical analysis at gale crater mars carbon dioxide is an essential atmospheric component in martian climate models that attempt to reconcile a faint young sun with planetwide evidence of liquid water in the noachian and early hesperian in this study we use mineral and contextual sedimentary environmental data measured by the mars science laboratory msl rover curiosity to estimate the atmospheric partial pressure of co2 pco2 coinciding with a long lived lake system in gale crater at 3 5 ga a reaction transport model that simulates mineralogy observed within the sheepbed member at yellowknife bay ykb by coupling mineral equilibria with carbonate precipitation kinetics and rates of sedimentation indicates atmospheric pco2 levels in the 10s mbar range at such low pco2 levels existing climate models are unable to warm hesperian mars anywhere near the freezing point of water and other gases are required to raise atmospheric pressure to prevent lake waters from being lost to the atmosphere thus either lacustrine features of gale formed in a cold environment by a mechanism yet to be determined or the climate models still lack an essential component that would serve to elevate surface temperatures at least locally on hesperian mars our results also impose restrictions on the potential role of atmospheric co2 in inferred warmer conditions and valley network formation of the late noachian

a warm or a cold early earth new insights from a 3 d climate carbon model abstract oxygen isotopes in marine cherts have been used to infer hot oceans during the archean with temperatures between 60 c 333 k and 80 c 353 k such climates are challenging for the early earth warmed by the faint young sun the interpretation of the data has therefore been controversial 1d climate modeling inferred that such hot climates would require very high levels of co2 2 6 bars previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the hadean and the archean here we revisit the climate and carbon cycle of the early earth at 3 8 ga using a 3d climate carbon model we find that co2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects however such high co2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts producing an efficient stabilizing feedback moreover the weathering of impact ejecta during the late heavy bombardment lhb would have strongly reduced the co2 partial pressure leading to cold climates and potentially snowball earth events after large impacts our results therefore favor cold or temperate climates with global mean temperatures between around 8 c 281 k and 30 c 303 k and with 0 1 0 36 bar of co2 for the late hadean and early archean finally our model suggests that the carbon cycle was efficient for preserving clement conditions on the early earth without necessarily requiring any other greenhouse gas or warming process

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

paleoclimate reconstruction of the prince creek formation arctic alaska during maastrichtian global warming abstract the upper cretaceous maastrichtian prince creek formation of arctic alaska 82 85 n paleolatitude preserves successions of alluvial paleosols that are used to estimate mean annual precipitation map mean annual temperature mat and meteoric water oxygen isotope composition for the prince creek formation the highest map estimates range between 1000 and 3900 mm yr and the lowest range between 350 and 1200 mm yr the precipitation variability derived from stable carbon isotope analysis occurs on a time scale of 104 years map values agree with previous interpretations from fossil pollen pedological features soil types vegetation composition sedimentological indicators and clamp estimates that suggest high precipitation amounts and high humidity despite the considerable uncertainty with mat proxies our values are consistent with warm month mean temperatures obtained previously from paleobotanical data the 18o of smectite from bentonite beds clusters around 5 0 vsmow the 18o of meteoric water calculated from bentonitic smectite is 23 vsmow assuming a mean annual temperature of 6 3 c which is slightly more 18o depleted than meteoric water calculated from pedogenic siderite in stratigraphically younger beds closer to the brooks range chronostratigraphic correlation with the lower cantwell formation denali national park alaska 64 n paleolatitude suggests higher precipitation rates and highly 18o depleted precipitation in the late cretaceous paleo arctic compared to central alaska these data are consistent with previous studies that suggest a warmer and moister greenhouse earth and an intensified hydrological cycle that enhanced latent heat transport resulting in increased rainout effects

the sturgeon falls paleosol and the composition of the atmosphere 1 1 ga bp a paleosol is exposed along the north bank of the sturgeon river some 25 km sw of baraga michigan the paleosol was developed on hydrothermally altered keweenawan basalt and is overlain by the jacobsville sandstone textures mineralogy and chemical composition change gradually upwards from unweathered metabasalt through the paleosol to the contact of the paleosol with the jacobsville sandstone many of these changes are similar to those in modern soils developed on basaltic rocks however k has clearly been added to the paleosol probably by solutions which had equilibrated with k feldspar in the jacobsville sandstone the keweenawan basalt was oxidized quite extensively during its conversion to greenstone during weathering the remaining fe2 was oxidized to fe3 and was retained in the paleosol the composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the o2 pressure to the co2 pressure in the atmosphere during the formation of the paleosol formula see text free o2 must have been present in the atmosphere 1 1 ga ago but its partial pressure could have been 10 3 times lower than in the atmosphere today

petrologic analysis and geochemistry of the late neogene early quaternary hardpan calcretes of western rajasthan india abstract calcretes formed over bedrock such as precambrian hornblende chlorite schist rhyolite and carbonaceous phyllites in western rajasthan india were studied for their mineralogical composition and geochemistry they form a distinct element of the quaternary landscape of western rajasthan india the study area is located within the nagaur churu jaipur tract of western rajasthan the purpose of this study was to determine the processes governing the development of hardpan calcretes and to evaluate the local and regional controls on their formation micromorphology included pedogenetic and groundwater features within the hardpan calcretes thickening of calcite laminae downward and tapering at the sideward edges around the unweathered minerals of quartz and feldspars indicated cumulative and compound pedogenesis which probably occurred locally and downward movement of carbonate solution and pore water carbonate solutions were probably derived from the upper horizons or surfaces bringing about the process of dissolution and recementation of individual laminae occurrence of fibrous palygorskite as coatings around the detrital grains and siderite oolitic and pisolitic in shape points to a subalkaline subacidic process of pedogenesis in a semi arid to arid climate stable 13c and 18o isotope data of the hardpan calcrete laminae vary between 0 3 to 1 5 and 5 9 and 1 5 respectively indicating their formation at or near surface capillary fringe probably supporting a thin column of soil the source of most of the calcite is groundwater however calcite nodule formation was largely dependent on pedogenic processes associated with evaporation evapotranspiration and or microenvironmental changes in ph and co2 partial pressure dust is also a major source for carbonate precipitation although it is commonly assumed that the powdery calcretes are younger in age compared to the more complex forms the occurrence of middle palaeolithic tools below the hardpan calcrete at roopangarh and dayalpura and above the hardpan calcretes at mitri genana and rol indicates that the morphology of calcretes is not a reliable indicator of age

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

refining a pedogenic carbonate co2 paleobarometer to quantify a middle miocene greenhouse spike abstract estimates of respired soil co2 concentration in paleosols are needed to calculate past atmospheric co2 levels from the difference between organic and carbonate carbon isotopic values in paleosols a new compilation of co2 concentrations late in the growing season in modern calcareous soils can be used to estimate respired soil co2 by subtracting atmospheric co2 concentrations furthermore respired co2 shows a significant relationship with depth to calcic horizon and this relationship can be used to estimate respired co2 concentrations in paleosols these values as well as estimates of 13c of atmospheric co2 from 13c of soil organic matter and of paleotemperature from bw horizon chemical composition refine the cerling co2 paleobarometer these refinements are applied to quantify the middle miocene greenhouse which has been controversial because it was not detected by marine alkenone and boron paleobarometers new estimates of middle miocene atmospheric co2 from paleosols in railroad canyon idaho reveal levels at 16 ma of 852 86 ppmv this is similar to predicted values by 2100 and 3 times pre industrial values pil 280 ppmv

ubiquitous occurrence of basaltic derived paleosols in the late archean fortescue group western australia abstract the 2 76 ga old mount roe basalt paleosols mr 1 and mr 2 recognized near the base of the 2 76 2 69 ga fortescue group in western australia represent some of the oldest definite examples of archean paleoweathering profiles the loss of fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the late archean atmosphere respectively however the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post weathering alteration over primary soil profiles here we report new exposures of the mr 1 paleosol as well as a number of new basalt derived paleosol occurrences distributed in the mount roe basalt and the 2 73 ga old kylena formations of the fortescue group we show that all these paleosols including mr 1 and mr 2 were strongly affected by post weathering reductive alteration nevertheless we discovered early lithologic units locally preserved within the hydrothermally altered paleosols which feature distinct chemical and mineralogic compositions these include i 13c depleted carbonaceous rich diaspore pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and ii green hard core material mostly composed of fe sericite associated with authigenic sphene and containing early relics of sulfate bearing iron rich smectite or berthierine we argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage while berthierine and sphene formed during diagenesis through the circulation of reductive fluids because the depletion of iron observed in the fortescue paleosols is not solely due to pedogenesis but also to post weathering alteration particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer recognition of the regional scale syn depositional alteration of the fortescue subaerial basalts over the north pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the pilbara craton during continental break up and rifting such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and or lacustrine systems favoring the development of microbial life in shallow waters

paleosol occurrences along the archean proterozoic contact in the aravalli craton nw india abstract the archean proterozoic contact apc comprises a large scale folded surface around udaipur in nw india the apc is exposed on both eastern and western flanks of a synclinorium formed during thick skinned thrusting of the archean basement and the paleoproterozoic cover units paleosol occurrences on both the limbs of this structure imply an originally continuous paleoweathering horizon and not individual pockets of paleosol as seen today preservation of paleosol sections along the archean proterozoic interface in the aravalli craton is strongly controlled by the structural grain complete paleosol sections are recognized in the eastern flank of the udaipur synclinorium in low strain regions with respect to the regional f1 and f2 folding in this sector the tulsi namla paleosol section represents an ancient lateritic weathering profile with an iron depleted pallid lower zone lithomarge and an iron enriched upper zone mottled zone to ferricrete original kaolinite in the saprolite and pallid zone has been transformed into kyanite during metamorphism and subsequently to white mica during retrograde overprint while fe rich clay and goethite were probably the primary minerals for the chloritoid and hematite which now constitute the upper zone at the western flank of the synclinorium the contact is heavily sheared and archean basement and proterozoic units show interfingering on a small scale here the paleosol sections are incomplete the weathering interval can be inferred by the stabilization of the craton at 2 5 ga and deposition of rift related sequences of the aravalli basin delwara formation at 2 15 ga the udaipur paleosols especially in the eastern flank represent the most complete archean paleoproterozoic paleosol sections in india

constraints on ocean carbonate chemistry and pco2 in the archaean and palaeoproterozoic one of the great problems in the history of earth s climate is how to reconcile evidence for liquid water and habitable climates on early earth with the faint young sun predicted from stellar evolution models possible solutions include a wide range of atmospheric and oceanic chemistries with large uncertainties in boundary conditions for the evolution and diversification of life and the role of the global carbon cycle in maintaining habitable climates increased atmospheric co 2 is a common component of many solutions but its connection to the carbon chemistry of the ocean remains unknown here we present calcium isotope data spanning the period from 2 7 to 1 9 billion years ago from evaporitic sedimentary carbonates that can test this relationship these data from the tumbiana formation the campbellrand platform and the pethei group exhibit limited variability such limited variability occurs in marine environments with a high ratio of calcium to carbonate alkalinity we are therefore able to rule out soda ocean conditions during this period of earth history we further interpret this and existing data to provide empirical constraints for carbonate chemistry of the ancient oceans and for the role of co 2 in compensating for the faint young sun

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

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

palaeoclimatology archaean atmosphere and climate arising from h ohmoto y watanabe k kumazawa nature 429 395 399 2004 see also communication from n h sleep h ohmoto y watanabe reply ohmoto et al 1 argue that carbon dioxide was abundant in the late archaean and early proterozoic atmosphere and that methane was probably scarce based on a reanalysis of the occurrence of siderite feco3 in ancient rocks here i consider several factors that may undermine their conclusions

the chemical index of alteration cia as a proxy for climate change during glacial interglacial transitions in earth history abstract the glacial interglacial transitions of ice ages are associated with large rapid changes in climate which potentially can be reconstructed from stratigraphic profiles of the chemical index of alteration cia here we present a case study based on high resolution cia profiles of neoproterozoic glacial deposits from south china paleolatitude 30 n as a record of the climate transition at the termination of the sturtian glaciation 663 ma drillcore zk2115 gaodi deposit exhibits a progressive upsection increase in mean cia over a 50 m interval from 58 range 52 65 note all ranges are given as 16th 84th percentiles in the synglacial upper tiesi ao formation to 67 range 64 69 in the deglacial basal datangpo formation cap carbonates and to 68 range 66 70 in the postglacial datangpo formation black shales a coeval section from the lijiawan deposit also exhibits an increasing cia trend upsection within shallow water mn carbonate facies these cia patterns show no relationship to lithology and are interpreted to reflect climatic warming and intensified chemical weathering during the sturtian deglaciation similar large increases in postglacial cia values are seen in paleoproterozoic and late paleozoic successions and at least moderate increases in cia values are recorded during warm stages of the late quaternary e g the bolling allerod and holocene relative to the cold stages e g the last glacial maximum and younger dryas the rapidity of cia changes in quaternary systems 1 to 10 kyr suggests that weathering intensity may have changed at similarly rapid rates in more poorly dated deep time glacial successions with potential implications for snowball versus slushball models of cryogenian glaciations these examples show that cia can be a robust proxy for climate changes i e enhanced chemical weathering intensity during glacial interglacial transitions of both ancient and recent ice ages

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

behavior of major and trace elements including ree during paleoproterozoic pedogenesis and diagenetic alteration of an archean granite near ville marie quebec canada abstract this paper presents a complete set of chemical data from a pre 2 0 ga weathering profile that formed on the archean granite near ville marie quebec examination of data from two sections of the paleosol indicates that na ca p and sr were lost from the paleosol whereas ti nb th zr hf and ta remained immobile during chemical weathering and all subsequent alteration rare earth mobility during genesis of the ville marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles average concentration of ree in both profiles is 20 40 lower than what is found in the fresh granite suggesting mobilization during pedogenesis hrees seem to be less mobilized than the lrees la sm n ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith gd yb n values remain unchanged in the weathering profile la yb n values in the weathering profile are quite variable and generally lower than that of the protolith eu eu and ce ce values are essentially the same as those of the protolith but two samples from the uppermost part of the profile have developed positive cerium anomaly of 0 3 relative to the protolith

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

anatomy of a sub cambrian paleosol in wisconsin mass fluxes of chemical weathering and climatic conditions in north america during formation of the cambrian great unconformity abstracta paleosol beneath the upper cambrian mount simon sandstone in wisconsin provides an opportunity to evaluate the characteristics of cambrian weathering in a subtropical climate having been

mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria filamentous and shrub like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca 1200 ma society cliffs formation baffin and bylot islands arctic canada are 400 m y older than previously reported examples in vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms ccms and is a direct ecophysiological link to atmospheric co 2 concentration ccms are induced in present day cyanobacteria under experimental conditions when pco 2 is below 0 36 10 times present atmospheric level pal society cliffs calcified cyanobacteria consequently imply pco 2 levels of 2 of 7 10 pal in the late mesoproterozoic combined petro graphic experimental and modeling results therefore suggest that mesoproterozoic pco 2 concentrations were not substantially different from phanerozoic values and were significantly less than previous estimates of up to 200 pal assuming 10 lower solar luminosity in the late mesoproterozoic pco 2 levels of 10 pal or less require the presence of additional greenhouse gases for maintenance of an ice free earth at 10 pal pco 2 methane concentrations of 100 200 ppm would have been sufficient to sustain warm earth surface conditions the low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of earth9s proterozoic biosphere

chromium isotope fractionation during oxidative weathering implications from the study of a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports detailed stable cr sm nd and rb sr isotope data for a ca 1 9 ga old subaerial weathering profile at schreiber beach ontario canada from which detailed major and trace element signatures and 18 o values were previously reported the weathering profile developed on neoarchean 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic 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 the aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough in a period following the great oxygenation event goe 2 45 ga ago and prior to the deposition of the gunflint cherts at 1 82 ga these iron rich cherts belong to one of the last regionally extensive banded iron formations bifs deposited before the 1 billion year long mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of bifs the gradual textural mineralogical and geochemical changes from unweathered basalts to strongly weathered hematite bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions the 53 cr value of unweathered pillow basalt cores 0 19 0 02 2 is within the range of mantle inventory values whereas weathered brown to green basalts soils exhibiting up to 30 lower cr concentrations compared to unweathered pillow cores are isotopically lighter 53 cr 0 35 0 11 in contrast red hematite rich basalts and hyaloclastites underlying the brown to green basaltic soils with the highest 18 o enrichment in the profile are isotopically heavier 53 cr 0 05 0 15 rb sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574 24 ma mswd 13 which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence burial of the gunflint basin the non correlation of k enrichment factors with 53 cr 18 o and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering instead correlations between 53 cr and 18 o and between 53 cr ce ce u and v indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re deposition at depth from high 18 o freshwaters partial re precipitation of heavy cr at depth can be explained on the basis of the weak positive correlation between 53 cr and fe enrichment factors it is seen as a consequence of the reduction of mobile cr vi and co precipitation as mixed fe iii cr iii oxhydroxides during contemporaneous oxidation of the fe 2 bearing groundwaters at depth a process similar to the one governing the deposition of most precambrian bifs redistribution of certain elements during the weathering process in this case rees is furthermore indicated by a correlation line in a sm nd isochron diagram defined by the schreiber beach data with a slope corresponding to an age of 1 93 0 19 ga mswd 26 an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality an oxidative atmosphere at 1 9 ga as implied by the results from the schreiber profile is furthermore supported by positively fractioned cr isotopes 53 cr from 0 06 to 0 39 recorded in the iron rich gunflint cherts directly above the palaeo weathered horizons at schreiber beach these values are interpreted to stem from a positively fractionated shallow seawater chromium composition at 1 88 ga potentially reflecting a continental run off characterized by positive 53 cr and are in accordance with cr isotope signatures in worldwide bifs including the gunflint iron formation our results show the potential of cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in earth s history

genesis of a giant paleoproterozoic strata bound magnesite deposit constraints from mg isotopes abstract giant strata bound magnesite deposits are absent in modern and most phanerozoic sedimentary environments but occur predominantly in precambrian strata these deposits may have formed directly through precipitation of evolved mg rich seawater in an evaporative shallow marine setting or alternatively by epigenetic hydrothermal replacement of the mg rich carbonate precursor to test these hypotheses we obtained the first mg isotope data from the world s largest strata bound magnesite deposit belt hosted by the ca 2 1 ga dashiqiao formation in northeast china the mg isotope compositions 26 mg of most magnesite ores in the huaziyu deposit are heavier 0 75 0 26 than most proterozoic sedimentary dolostones the mg isotope compositions and major and trace element data indicate that the magnesites are probably not of hydrothermal origin instead a mg rich carbonate precursor precipitated from evaporating seawater in a semi closed system diagenetic brines altered the mg rich carbonate precursor to magnesite subsequently recrystallization during regional metamorphism produced coarsely crystalline and saddle magnesite these interpretations are consistent with the geological features and other geochemical data element concentrations and c and o isotopes for the magnesite ores hence we interpret the formation of the huaziyu magnesite deposit to be dominated by evaporative sedimentation and brine diagenesis

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

theoretical constraints on oxygen and carbon dioxide concentrations in the precambrian atmosphere simple one dimensional climate models suggest that carbon dioxide concentrations during the archean must have been at least 100 1000 times the present level to keep the earth s surface temperature above freezing in the face of decreased solar luminosity such models provide only lower bounds on co2 so it is possible that co2 levels were substantially higher than this and that the archean climate was much warmer than today periods of extensive glaciation during the early and late proterozoic on the other hand indicate that the climate at these times was relatively cool to be consistent with climate models co2 partial pressures must have declined from approximately 0 03 to 0 3 bar around 2 5 ga ago to between 10 3 and 10 2 bar at 0 8 ga ago this steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data which implies that pco2 did not change appreciably during that time oxygen was essentially absent from the earth s atmosphere and oceans prior to the emergence of a photosynthetic source probably during the late archean during the early proterozoic the atmosphere and surface ocean were apparently oxidizing while the deep ocean remained reducing an upper limit of 6 x 10 3 bar for po2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean the establishment of oxidizing conditions in the deep ocean marked by the disappearance of banded iron formations approximately 1 7 ga ago permitted atmospheric oxygen to climb to its present level o2 concentrations may have remained substantially lower than today however until well into the phanerozoic

a model for late archean chemical weathering and world average river water abstract interpretations of the geologic record of late archean near surface environments depend very strongly on an understanding of weathering and resultant riverine transport to the oceans the late archean atmosphere is widely recognized to be anoxic p o 2 g 10 5 to 10 13 bars p h 2 g 10 3 to 10 5 bars detrital siderite feco3 pyrite fes2 and uraninite uo2 in late archean sedimentary rocks also suggest anoxic conditions however whether the observed detrital minerals could have been thermodynamically stable during weathering and riverine transport under such an atmosphere remains untested similarly interpretations of fluctuations recorded by trace metals and isotopes are hampered by a lack of knowledge of the chemical linkages between the atmosphere weathering riverine transport and the mineralogical record in this study we used theoretical reaction path models to simulate the chemistry involved in rainwater and weathering processes under present day and hypothetical archean atmospheric boundary conditions we included new estimates of the thermodynamic properties of fe ii smectites as well as smectite and calcite solid solutions simulation of present day weathering of basalt calcite by world average rainwater produced hematite kaolinite na mg saponite and chalcedony after 10 4 moles of reactant minerals kg 1 h2o were destroyed combination of the resultant water chemistry with results for granitic weathering produced a water composition comparable to present day world average river water warw in contrast under late archean atmospheric conditions p co 2 g 10 1 5 and p h 2 g 10 5 0 bars weathering of olivine basalt calcite to the same degree of reaction produced kaolinite chalcedony and na fe ii rich saponite late archean weathering of tonalite trondhjemite granodiorite ttg formed fe ii rich beidellite and chalcedony combining the waters from olivine basalt and ttg weathering resulted in a model for late archean warw with the composition na ca2 fe2 mg2 cl hco 3 sio2 aq the ph of the water was 6 3 and it is much richer in hco 3 and in mg fe relative to ca na compared to present day warw at higher ph2 g e g 10 4 0 bars organic acid anions could be metastable our results are consistent with the thermodynamic stability of fe ii clays pyrite uraninite and under some conditions siderite during weathering and riverine transport overall our results provide a basis for assessing the formation of organic hazes and the mobility of trace elements and nutrients due to fluctuations of the late archean atmosphere

quantitative geochemical approach to pedogenesis importance of parent material reduction volumetric expansion and eolian influx in lateritization abstract using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock through calculation of net mass fluxes through the laterite soils in mali west africa we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density contributes only a very minor fraction of the enrichment of al fe si and au instead we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features at the same sample depths that accumulation reaches extreme values we show that volumetric expansion in excess of 200 has occurred locally we infer that these spatially coincidental zones of mass influx of rock forming metals fe al si and also au with dilational hyperstrains result from a mutually reinforcing mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink swell cycles and root growth and decay we propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded we speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids consequently progressive hyperstrains accumulate above this critical depth by the long term influence of a proposed translocational wedge of chemically resistant minerals against which numerous generations of plant roots have exerted stresses eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted instead the source region is largely the existing regolith column itself which releases local material and in addition is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

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

early triassic terrestrial tetrapod fauna a review abstract the permian triassic mass extinction ptme ca 252 mya was one of the most severe biotic crises of the phanerozoic eliminating 90 of marine and terrestrial species this was followed by a long period of recovery in the early and middle triassic which revolutionised the structure of both marine and terrestrial ecosystems triggering the new ecosystem structure of the mesozoic and cenozoic entire new clades emerged after the mass extinction including decapods and marine reptiles in the oceans and new tetrapods on land in both marine and terrestrial ecosystems the recovery is interpreted as stepwise and slow from a combination of continuing environmental perturbations and complex multilevel interaction between species in the new environments as ecosystems reconstructed themselves here we present a review of early triassic terrestrial tetrapod faunas geological formations and outcrops around the world and provide a semi quantitative analysis of a data set of early triassic terrestrial tetrapods we identify a marked regionalisation of early triassic terrestrial tetrapods with faunas varying in both taxonomic composition and relative abundance according to palaeolatitudinal belt we reject the alleged uniformity of faunas around pangaea suggested in the literature as a result of the hot house climate in addition we can restrict the tetrapod gap of terrestrial life in the early triassic to palaeolatitudes between 15 n and about 31 s in contrast to the earlier suggestion of total absence of tetrapod taxa between 30 n and 40 s there was fairly strong provincialism following the ptme according to cluster analysis of a taxon presence matrix entirely consistent with early triassic palaeobiogeography unexpectedly the overall pattern for early triassic terrestrial tetrapod faunas largely reflects that of the late permian suggesting that the recovery faunas in the early triassic retained some kind of imprint of tetrapod distributions according to palaeogeography and palaeoclimate despite the near total extinction of life through the ptme

the enigma of neoproterozoic giant ooids fingerprints of extreme climate geologists have documented at least fourteen occurrences of giant ooids a geologically rare type of carbonate allochem in neoproterozoic successions at low paleo latitudes recent experiments a

evidence for climate shifts in the 2 0ga upper makgabeng formation erg south africa abstract eolian ergs and associated environs are sensitive to short and long term climatic changes in one of the oldest erg deposits the 2 0 ga makgabeng formation vertical changes in facies associations reflect one of the earliest records of short term climatic shifts in a continental setting the makgabeng formation is separated into lower and upper erg deposits by a playa or saline pan deposit the lower erg deposit consists of dune sets with thin lenses of dry and deflationary interdune strata that transition vertically to thicker damp to wet interdune strata a laterally persistent playa deposit in the middle of the section consists of mudstone with deep and shallow penetrating mud cracks and subordinate siltstone and sandstone interbeds above this lower mudstone interval the playa strata are sandy overlying the playa deposit the upper erg deposit consists of thick eolian sets with thin lenses of dry interdune deposits grain size change near the top of the upper erg deposit corresponds with the appearance of fluvial sheet flood eolian cross beds sculpted by mass flows and thin playa deposits vertical stacking of facies associations demonstrates temporal shifts in precipitation and fluctuating water tables fluvial and playa deposits record high water tables whereas low water tables are reflected in the core erg the transition from a lower to higher water table is recorded by wet interdune interspersed within the dune strata toward the top of this interval rapid climatic amelioration occurred near the termination of the makgabeng erg resulting in impingement of ephemeral river systems development of playas and generation of massive sand flows this 2 0 ga erg demonstrates the impact of climate change on erg development resulting from shifts in monsoonal impingement through time

the paleoproterozoic baraboo paleosol revisited quantifying mass fluxes of weathering and metasomatism chemical climofunctions and atmospheric pco2 in a chemically heterogeneous protolith abstract the paleoproterozoic baraboo paleosol is the most chemically mature precambrian paleosol in the lake superior region having formed under the influence of a warm humid climate in a region of subdued topography we have determined the chemical variability among six samples of granitic protolith and found that such variability is inherited in the weathering profile to minimize the effect of sample heterogeneity on various geochemical calculations transformed compositions of saprolite and regolith were generated by obtaining linear least squares fits to the concentrations of sio2 tio2 al2o3 and zr through the weathering profile and summing the totals to 100 fortunately use of measured or transformed compositions makes little difference in calculation of mass fluxes however transformed compositions allow for clearer comparisons among paleosols and are essential for determining certain paleosol parameters such as chemical depletion fraction calculations of total and reduced mass fluxes chemical index of alteration feldspar index of weathering and chemical depletion fraction demonstrate that the baraboo paleosol is chemically more mature than the paleoproterozoic pronto and cambrian timna paleosols but less mature than modern subtropical and tropical soils estimates of mean annual temperature matppm and mean annual precipitation mapppm for baraboo are 14 3 19 2 9 4 c and 1177 1695 658 mm yr 1 confirming the previous interpretation of a warm humid climate although the level of atmospheric pco2 is poorly constrained due to the uncertainty in duration of weathering and possibility of regolith removal by erosion pco2 is calculated to be 4 8 pre industrial atmospheric level 280 ppm for 100 000 years of weathering which is consistent with the pco2 level at 1 7 ga predicted by the weak sun model

chemostratigraphy of the late mesoproterozoic atar group taoudeni basin mauritania muted isotopic variability facies correlation and global isotopic trends a b s t r a c t sedimentary strata of the atar el mreiti groups taoudeni basin mauritania are characterized by car bon isotope values that fall largely between 2 5 and 4 0 and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from 1 2 ga to 1 1 ga chemostratigraphic results are consistent with recent re os dates of 1 1 ga for atar el mreiti group strata rooney et al 2010 and support an emerging dataset that identifies the late mesoproterozoic as an isotopically distinctive interval in earth history the relatively low amplitude carbon isotope variabil ity limits the use of chemostratigraphy in detailed correlation during this time interval in this study we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of deposi tional facies to construct a rigorous and testable model for the intrabasinal correlation of atar and el mreiti group strata data support a depositional model that results in broadly uniform stratigraphic thicknesses across the west african craton with predominantly stromatolitic facies of the atar group mauritania and the hank and dar cheikh groups algeria representing deposition on shallow water craton margins and clayey carbonate and shale facies of the el mreiti group mauritania representing deposition on a broad shallow water epicratonic platform within this stratigraphic framework regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely asso ciated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean similarly distinct differences in trace element composition recorded in multilayered marine cements within deeper water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox gradients in the water column we use this multifaceted approach to test a recent hypothesis that suggests correlation of atar el mreiti group strata with glaciogenic strata of the vazante group sao francisco craton critically preservation of shallow water carbonate and evaporite strata across more than 1200 km of the predominantly epicra tonic taoudeni basin with minimal evidence for high amplitude sea level change is consistent with inferences of late mesoproterozoic highstand conditions in a global greenhouse climate such charac teristics are inconsistent with suggestions of a time equivalency with glaciogenic strata of the vazante group sao francisco craton despite broad similarities in re os geochronology and carbon isotope com position together these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution 2012 elsevier b v all rights reserved

from a black to a gray box a mass balance interpretation of pedogenesis we utilize chemical elements as tracers in a mass balance analysis that provides functional relationships among soil chemical composition bulk density and volume change in relation to parent material these analytical functions are based on the principle of conservation of mass and include a term quantifying mass flux into out of the soil and between horizons we apply the technique to the oldest member of a chronosequence developed on marine terraces in northern california it is an alfisol that evolved from beach sand to its present state nearly one third of its weight is composed of secondary clay minerals in about 240 ky aside from large increases in organic carbon desilication is the dominant factor in soil evolution 29 50 83 g cm 2 si was leached from the beach sand during pedogenesis the rate of desilication is roughly 2 1 t km 2 yr 1 0 21 g cm 2 ky 1 an order of magnitude slower than that implied by the si denudation rate calculated for the mattole river watershed weathering of primary minerals and synthesis of secondary minerals is relatively well advanced suggesting that the rate of desilication may be declining the local beach is composed of quartz and sodic plagioclase with smaller amounts of chlorite mica and kaolinite the soil has substantially different mineralogy sand is dominated by quartz and clay is dominated by kaolinite halloysite chloritic intergrades and gibbsite bases are also leached though the total mass was much less 57 2 55 g cm 2 of na in the beach sand was lost as plagioclase weathered by focusing on elemental and mineralogical gains and losses we emphasize the essential connection between the pedologic environment and the external hydrochemical environment

planum boreum basal unit topography mars irregularities and insights from sharad shallow radar investigations of planum boreum mars basal unit bu deposit have revealed multiple reentrants morphologic irregularities and thickness trends that differ from those of the overlying north polar layered deposits we present detailed subsurface maps for these features and offer explanation for genesis of the deposit s morphologic asymmetry expressed in different erosional characteristics between 0 e 180 e and 180 e 360 e additionally this work revealed a depression in the basal unit that may have provided a site for spiral trough initiation interpretations of the findings suggest that antecedent bu topography has a marked impact on modern morphology and that aeolian forces have been the dominant driver of polar deposit accumulation since at least the end of rupes unit emplacement we find no results requiring explanation beyond common martian surface processes including aeolian erosion and impact armoring to add to the detailed morphologic study of the bu we mapped the variability of the bu radar reflection character combining generalized katabatic wind flow with the radar mapping results suggests that rupes unit material sourced the younger cavi we present clear evidence that while compositionally distinct from the overlying layered deposits the bu and its morphology are intimately linked to the morphology of the north polar layered deposits combining geologic evidence with paleoclimate modeling the deposits contain evidence for a long history of aeolian emplacement and modification

are there ancient soils in the 3 7 ga isua greenstone belt greenland abstract a lens of black schist within 3 7 ga quartzites of greenland may be earth s oldest known alluvial paleosol the suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite but it has a truncated top above dark gray grading down to gray color ptygmatically folded surface cracks filled with silt grains and large sand crystals unusual for sedimentary or metamorphic rocks the paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends and by chemical analysis showing molar weathering ratios and ree distribution like those of soils the schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the isukasia area the protolith can be reconstructed as a saponite clay with a salt rich horizon of kieserite like other acid sulfate paleosols of the early earth models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 510 ppm o2 and 820 201 ppm co2 and humid cool temperate paleoclimate the profile has organic 13cpdb consistently of 24 2 to 27 4 and modest raleigh distillation near the top similar consistent values and trends are produced by decay of organic matter in living soils but biotic carbon isotopic composition of sediments is erratic from bed to bed and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle thus life in this very ancient soil is not precluded by our analyses but ultrastructural and geochemical testing of carbon particles would further test this hypothesis

irrefutable detrital origin of witwatersrand gold and evidence of eolian signatures toroidal shaped gold particles extracted from a sample of the basal reef orebody in the welkom gold field that contained crossbedded gold have been recognized in thin section demonstrating that the form in question is not an artifact of extraction but is a genuine detrital form similar forms have been recovered in great abundance from an eolian paleosurface in yakutia and from a recent storm beach placer in new zealand indicating the likelihood that eolian processes produced these forms this has been confirmed by means of wind tunnel experiments toroids in various states of preservation have been extracted from a number of other witwatersrand paleoplacers where there is an association with ventifacts indicating that wind deflation might have played a role in concentrating gold from placer sand deposits

weathering during the great oxidation event fennoscandia arctic russia 2 06 ga ago abstract the establishment of an oxygen rich atmosphere between 2 4 and 2 0 billion years ago ga is one of the defining events in earth history however the nature of the transition from an anoxic atmosphere is poorly resolved with interpretations relying heavily on empirical evidence that is poorly calibrated and not supported by numerical modeling in an attempt to characterize the nature timing and impact the great oxidation event goe had on earth s surface the fennoscandian arctic russia drilling early earth project far deep targeted the archean proterozoic transition exposed in the eastern part of the fennoscandian shield far deep recovered more than 400 m of the highly oxidized 2 06 ga kuetsjarvi volcanic formation kvf which is potentially a manifestation of when atmospheric oxygen levels exceeded the ultimate proterozoic steady state identifying the timing and extent of oxidation within the kvf are the primary objectives of this study the presence of secondary hematite within fractures and amygdales indicates that the elevated oxidation state of the kvf is not a primary feature of the rocks immediately below the erosional surface marking the top of the kvf the degree of iron oxidation scales with the degree of chemical weathering implying that oxidation of the kvf is the result of interaction with oxygen charged groundwaters during subaerial exposure the incorporation of rounded clasts with conspicuous hematite rinds into the overlying kolosjoki sedimentary formation indicates that the oxidation of the kvf occurred prior to its deposition the oxidized regolith recovered in the far deep drill cores is more oxidized for the degree of weathering observed compared to older precambrian and archean aged paleosols but not anomalously so compared to younger proterozoic paleosols this observation suggests that the kuetsjarvi regolith was created during the progressive increase of po2 towards the proterozoic steady state using a previously published mass balance model to infer paleoatmospheric pco2 and po2 we are unable to provide any useful constraint on the paleoatmospheric pco2 and po2 levels we estimate that the paleoatmospheric pco2 and po2 levels ca 2 06 ga ago in fennoscandia were between 0 4 and 6 500 times present atmospheric level pal and 1 10 3 and 12 times pal respectively the wide range in the calculated pco2 and po2 levels is the result of the pco2 model calculations being extremely sensitive to key parameters that are nearly impossible to estimate accurately for ancient paleosols e g the annual precipitation rate depth to the groundwater table and the amount of time it took the soil to form

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

texture contrast profile with stonelayer in the cape peninsula south africa autochthony and polygenesis article i nfo article history received 21 october 2013 accepted 26 january 2014 available online xxxx

o2 constraints from paleoproterozoic detrital pyrite and uraninite redox sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the archean and early paleoproterozoic atmosphere today these minerals are rapidly chemically weathered within short transport distances prior to the rise of oxygen low o2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes after the rise of oxygen redox sensitive detrital grains effectively vanish from the sedimentary record to get a better understanding of the timing of this transition we examined sandstones recorded in a scientifi c drill core from the south african 2 415 ga koegas subgroup a mixed siliciclastic and iron formation bearing unit deposited on the western deltaic margin of the kaapvaal craton in early paleoproterozoic time we observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section indicating the rise of oxygen is younger than 2 415 ga to better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain earth surface redox conditions we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental o2 concentrations even conservative model calculations for deltaic depositional systems with suffi cient transport distances approximately hundreds of kilometers show that redox sensitive detrital grains are remarkably sensitive to environmental o2 concentrations and they constrain the archean and early paleoproterozoic atmosphere to have 3 2 10 5 atm of molecular o2 these levels are lower than previously hypothesized for redox sensitive detrital grains but they are consistent with estimates made from other redox proxy data including the anomalous fractionation of sulfur isotopes the binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric o2 concentrations rose substantially at this time and were never again suffi ciently low 0 01 atm to enable survival and preservation of these grains in short transport systems

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

a coupled carbon silicon cycle model over earth history reverse weathering as a possible explanation of a warm mid proterozoic climate abstract the balance between carbon outgassing and carbon burial controls earth s climate on geological timescales carbon removal in carbonates consumes both atmospheric carbon and ocean carbonate alkalinity sourced from silicate weathering on the land or seafloor reverse weathering rw refers to clay forming reactions that consume alkalinity but not carbon if the cations of alkalinity end up in clay minerals rather than in the carbonates carbon remains as atmospheric co 2 warming the climate higher silicate weathering fluxes and warmer temperatures are then required to balance the carbon cycle it has been proposed that high dissolved silica concentrations resulting from the absence of ecologically significant biogenic silica precipitation in the precambrian drove larger rw fluxes than today affecting the climate here we present the first fully coupled carbon silica cycle model for post hadean earth history that models climate evolution self consistently available as open source code rw fluxes and biogenic silica deposition fluxes are represented using a sediment diagenesis model that can reproduce modern conditions we show that a broad range of climate evolutions are possible but most plausible scenarios produce proterozoic warming 5 k relative to without rw which could help explain the sustained warmth of the proterozoic despite lower insolation rw in the archean is potentially more muted due to a lower land fraction and sedimentation rate key model uncertainties are the modern reverse weathering flux the rate coefficient for rw reactions and the solubility of authigenic clays consequently within the large uncertainties other self consistent scenarios where proterozoic rw was unimportant cannot be excluded progress requires better constraints on parameters governing rw reaction rates including explicit consideration of cation limitations to precambrian rw and perhaps new inferences from si or li isotopes systems