equatorial aridity in western pangea lower permian loessite and dolomitic paleosols in northeastern new mexico u s a abstract lower permian strata have been extensively cored in the subsurface of the bravo dome field northeastern new mexico analysis of core indicates that these strata consist of conglomeratic and sandy fluvial deposits and volumetrically significant eolian silt loessite fluvial facies dominate the lower half of the study interval and include matrix supported massive conglomeratic debris flow units and laminated arkosic sandstone whereas loessite dominates the upper half of the study section and consists of massive well sorted quartzose siltstone that locally reaches thicknesses as much as 120 m in the greater study region paleosols are present throughout the study interval and consist of protosols and dolosols commonly exhibiting vertic features dolomite that is interpreted to be of pedogenic origin is an unusual but volumetrically significant component in these paleosols paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial 3 8 latitudes but depositional and pedogenic evidence both suggest seasonally wet to markedly arid conditions from early wolfcampian to early leonardian time the loessite covers a substantial area 6000 km2 making this the largest pre cenozoic loess accumulation yet documented this is significant because loess generally suggests arid to semiarid conditions intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape stability which we relate to high frequency oscillation between dry and slightly wetter conditions possibly attributable to glacial interglacial climatic conditions that prevailed at low latitudes at a lower frequency the evolution from a predominance of fluvial to primarily eolian strata in tandem with changes in pedogenic character reflect a long term aridification for the study interval these data corroborate independent inferences of monsoon induced equatorial aridity in western pangea and help constrain the timing of the zonal to monsoonal transition to earliest permian time

pedogenic origin of dolomite in a basaltic weathering profile kohala peninsula hawaii we document stoichiometric dolomite occurring in a nonsaline quaternary soil on the kohala peninsula northwestern hawaii geologic constraints and geochemical and isotopic data confirm that this dolomite is not the result of marine influence or wind blown dust the strontium isotopic composition of the dolomite 87sr 87sr 0 7045 0 7048 is indicative of its derivation primarily from the weathering of basaltic parent material rather than from meteoric water or seawater infiltration of soil waters with elevated mg ca 1 derived from alteration of ferromagnesian minerals such as olivine likely led to dolomitization of early precipitated soil calcite and or to direct dolomite precipitation in the profile this demonstrates that well ordered dolomite can form in a nonmarine environment at temperatures 100 c without undergoing burial diagenesis

non marine permian biostratigraphy and biochronology during the permian the single supercontinent pangaea stretched from pole to pole early permian glacial deposits are found in southern gondwana along the sutures of pangaea mountain ranges towered over vast tropical lowlands interior areas included dry deserts where dune sands accumulated gypsum and halite beds document the evaporation of hot shallow seas that formed the most extensive salt deposits in the geological record the permian period 251 to 299 ma encompasses nine ages stages arranged into three epochs series most of the permian marine timescale has been defined by global stratotype sections and points for the stage boundaries this volume presents new data regarding the biostratigraphy and biochronology of the non marine permian and provides a basis for temporally ordering permian geological and biotic history on land and correlating that history to events in the marine realm

precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

middle late permian mass extinction on land the end permian mass extinction has been envisaged as the nadir of biodiversity decline due to increasing volcanic gas emissions over some 9 million years we propose a different tempo and mechanism of extinction because we recognize two separate but geologically abrupt mass extinctions on land one terminating the middle permian guadalupian at 260 4 ma and a later one ending the permian period at 251 ma our evidence comes from new paleobotanical paleopedological and carbon isotopic studies of portal mountain antarctica and comparable studies in the karoo basin south africa extinctions have long been apparent among marine invertebrates at both the end of the guadalupian and end of the permian which were also times of warm wet greenhouse climatic transients marked soil erosion transition from high to low sinuosity and braided streams soil stagnation in wetlands and profound negative carbon isotope anomalies both mass extinctions may have resulted from catastrophic methane outbursts to the atmosphere from coal intruded by feeder dikes to flood basalts such as the end guadalupian emeishan basalt and end permian siberian traps

the origin of low δ18o granites and related rocks from the seychelles the 750 ma seychelles granites have whole rock 18o values that range from 1 2 to 7 5 differences in 18o values between quartz feldspar and whole rock for samples that have a range of 18o values suggest that these minerals are in magmatic equilibrium and that the whole rock 18o values were little affected by post crystallization interaction with fluids two suites of granites the mahe type and the praslin type have previously been recognized on the basis of chemical and radiogenic isotope composition the former have a mean whole rock 18o value of 5 25 0 65 1 whereas the latter are much more variable with a mean whole rock 18o value of 3 00 3 27 biotite and amphibole separates from both granite types range in d from 69 to 116 and show a positive correlation with whole rock 18o values dolerite dykes that intruded the granites during or shortly after their crystallization also have whole rock 18o values mean 2 24 1 93 lower than that expected for mantle derived basaltic rocks but these values appear to be the result of fluid rock interaction we suggest that the mahe type granites are derived mainly from juvenile mafic to intermediate crust with normal d and 18o values whereas the praslin type granites are mixtures of this source and older crust that acquired its low d and 18o values by extensive interaction with meteoric water at high temperature it is unlikely that meteoric water sufficiently depleted in d and 18o was available at 750 ma because of the relatively low latitude of the region at that time we suggest that alteration of the source took place significantly before production of the granite magmas depletion in 18o of the seychelles granites does not necessarily require a regional extensional tectonic setting at 750 ma as has been proposed

tetrapod localities from the triassic of the se of european russia abstract fossil tetrapods amphibians and reptiles have been discovered at 206 localities in the lower and middle triassic of the southern urals area of european russia the first sites were found in the 1940s and subsequent surveys from the 1960s to the present day have revealed many more broad scale stratigraphic schemes have been published but full documentation of the rich tetrapod faunas has not been presented before the area of richest deposits covers some 900 000 km 2 of territory between samara on the river volga in the nw and orenburg and sakmara in the sw continental sedimentary deposits consisting of mudstones siltstones sandstones and conglomerates deposited by rivers flowing off the ural mountain chain span much of the lower and middle triassic induan olenekian anisian ladinian the succession is divided into seven successive svitas or assemblages kopanskaya induan staritskaya kzylsaiskaya gostevskaya and petropavlovskaya all olenekian donguz anisian and bukobay ladinian this succession comprising up to 3 5 km of fluvial and lacustrine sediments documents major climatic changes at the beginning of the early triassic arid zone facies were widely developed aeolian piedmont and proluvium these were replaced by fluvial facies with some features indicating aridity at the end of the middle triassic deltaic and lacustrine marsh formations were dominant indicating more humid conditions the succession of early to mid triassic tetrapod faunas documents the recovery of life after the end permian mass extinction the earliest faunas consist only of small aquatic tetrapods in low diversity low abundance assemblages climbing the succession through the early triassic more terrestrially adapted tetrapods appear and larger herbivorous and carnivorous reptiles come to dominate in the mid triassic as ecosystems were rebuilt

palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information much of the work performed in this area has focused on either of two types of terrestrial carbonate palustrine carbonates or calcretes pedogenic and groundwater yet their simultaneous study shows there may be a gradual transition from one form to the other revealing the interplay between pedogenic sedimentary and diagenetic processes three main factors control the formation of these carbonates the position of the water table the host rock and the period of sub aerial exposure in pedogenic calcretes precipitation of carbonate takes places mostly in the vadose zone above the water table and within a previous host rock or sediment in groundwater calcretes the precipitation of carbonate also occurs within a previous host rock and around the groundwater table in palustrine carbonates however the precipitation of lime mud occurs in a lacustrine water body palustrine carbonates necessarily form on previous lacustrine mud whereas both types of calcretes may form on any type of sediment or soil the sub aerial exposure time needed to form palustrine carbonates may by relatively short even a season whereas pedogenic calcretes need more time several years to millions of years groundwater calcretes do not form on the topographic surfaces so there is no need of sub aerial exposure however stable surfaces favour the development of thick groundwater calcretes small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features causing difficulties in the interpretation of these carbonates the formation of these carbonates is controlled by palaeoenvironmental factors both commonly form in semi arid climates arid climates are also suitable for calcretes but sub humid conditions are more suitable for palustrine carbonates more indications of climatic conditions may be obtained through the analysis of the d18o content of both calcretes and palustrine carbonates and from the depth of the horizon containing carbonate nodules in pedogenic calcretes vegetation is also important in the formation of these types of carbonates data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13c signal of the primary carbonates which in pedogenic carbonates has also been used to estimate atmospheric pco2 during the phanerozoic these terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins their presence and characteristics can be used as indicators of aggradation subsidence or accommodation rates and therefore as indicators of different tectonic regimes even though the study of these carbonates has notably increased in recent years much less is known about them than about marine carbonates presently there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins with a need to include the carbonates analysed in this paper

timing of mammal like reptile extinctions across the permian triassic boundary in south africa the rate timing and pattern of change in different regions and paleoenvironments are critical for distinguishing among potential causes for the permian triassic p t extinction carbon isotopic stratigraphy can provide global chronostratigraphic control we report a large 13 c excursion at the p t boundary and no long term permian 13 c trends for samples from the interior of pangea stratigraphic gaps between available samples limit the resolution of our 13 c curve but the excursion is within a 15 m thick zone of overlap between permian and triassic taxa sedimentological and taphonomic observations demonstrate that this 15 m interval does not represent geologically instantaneous deposition together these data support a rapid and globally synchronous p t event but suggest that it occurred over a geologically resolvable interval of time

groundwater dolocretes in a lake marginal environment an alternative model for dolocrete formation in continental settings danian of the provence basin france in the long continental history of the provence basin which extended from santonian to oligocene times a major period of palaeosol development occurred in the danian dolocretes developed within floodplain silts and partly from palustrine limestones dolocretes are overlain by pedogenic facies calcretes and palustrine limestones gradational lower and upper limits of profiles succession of nodular coalescent and massive horizons the epigenesis of quartz by dolomite the unimodal crystal size and the euhedral dolomite fabric as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone the exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering the term halo dolocrete is proposed to describe this type of dolocrete deposit subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization calcitization was either fabric destructive type i or fabric preserving type ii and took place during very early diagenesis i e concomitant with calcrete formation or palustrine limestone deposition fabric destructive calcitization is attributed to a drop in the lake level when the upper part of the dolocrete was subjected to vadose zone processes whereas the fabric preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level

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

isotope excursions and palaeotemperature estimates from the permian triassic boundary in the southern alps italy abstract the end permian mass extinction event coincides with a dramatic negative shift in both the 13c and the 18o isotope record at or near the extinction horizon from multiple marine and terrestrial sections round the globe this study has identified the isotope excursions in a shallow water carbonate dominated section at val brutta in the southern alps high resolution bulk rock sampling revealed that there is an average 13ccarb negative shift of 4 and a 18ocarb shift of 6 this is similar to the isotope record at other nearby localities this section like many others shows a degree of recrystallisation which hampers absolute palaeotemperature calculations however some brachiopod fossils including specimens of comelicania and janiceps found within the beds that contain the bulk rock anomaly appear to be remarkably well preserved and still retain their internal structure and the primary isotopic variation across the shell as such they have been successfully analysed and the data used to produce a palaeotemperature estimate at the time of the isotope anomaly of between 26 and 29 c for latest permian

magnetostratigraphy of permian triassic boundary sequences in the cis urals russia no evidence for a major temporal hiatus during the last five years there has been considerable doubt over the age of the continental uppermost permian russian stages the kazanian and tatarian traditionally they have been regarded as late permian but were re dated as middle permian in the 2004 international time scale despite fossil evidence that the tatarian at least is late permian these debated ages are tested by magnetostratigraphic study of five sections spanning the permian triassic boundary ptb of the se urals in the orenburg region of russia the upper permian and lower triassic of this region have a well documented vertebrate fauna whose evolution has a significant bearing on our understanding of the ptb mass extinction event if the tatarian is viewed as mid permian then the late permian in russia is marked by a 9 10 ma stratigraphic gap the palaeomagnetic data yield a distinct series of polarity zones that provide clear local and regional correlation and are readily tied to a recently compiled global magnetostratigraphic record on the basis of this correlation the sampled sections span the upper guadalupian to induan stages without any obvious break so confirming the traditional view that the tatarian is late permian in age anomalies in the magnetic inclination are consistent with sediment compaction inclination shallowing a common phenomenon of red beds but declination anomalies between these sites and elsewhere in russia may suggest localised vertical axis rotation

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

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

mid eocene alluvial lacustrine succession at gebel el goza el hamra shabrawet area ne eastern desert egypt facies analysis sequence stratigraphy and paleoclimatic implications abstract this study aims to provide the depositional facies sequence stratigraphic and paleoclimatic characteristics of the mid eocene bartonian continental succession exposed at gebel el goza el hamra shabrawet area ne eastern desert egypt the studied succession consists of siliciclastic rocks followed upward by carbonate rocks detailed field observation and petrographic investigation indicate accumulation in floodplain dominated alluvial and shallow lacustrine systems the floodplain dominated alluvial facies 45 m thick is composed mainly of carbonate nodules bearing mottled mudrock with subordinate sandstone and conglomerate beds the conglomerate and pebbly sandstone bodies interpreted as ephemeral braided channel deposits the massive laminated planner cross bedded fine to medium grained sandstone bodies interlayered within mudstone reflect sheet flood deposits the mudrocks associated with paleosols represent distal floodplain deposits the shallow lacustrine facies 15 m thick is made up of an alternation of marlstone micritic limestone dolostone and mudrock beds with charophytes and small gastropods both the alluvial and lacustrine facies show evidence of macro and micro pedogenic features pollen assemblages stable 18 o and 13 c isotopes and paleopedogenic features reflect prevalence of arid to semi arid climatic conditions during the bartonian the sequence stratigraphic framework shows an overall fining upward depositional sequence consisting of low and high accommodation systems tracts last hast and is bounded by two sequence boundaries sb 1 sb 2 conglomerate and pebbly sandstone deposits braided channel and sheet flood deposits of the lower part of the alluvial facies reflect a last mudrock and silty claystone facies distal floodplain deposits of the upper part of alluvial facies and its overlying lacustrine facies correspond to a hast the last hast and sb were formed during different accommodation to sediment supply a s ratio phases the variation in a s ratios was mainly controlled by sea level change as well as by local tectonic subsidence and uplift of the basin coincident with the reactivation of the syrian arc system during the bartonian

deep infiltration of surface water during deformation evidence from a low δ18o shear zone at koegel fontein namaqualand south africa abstract the 144 133ma koegel fontein igneous complex is notable for the high proportion of magmatic rocks with very low 18o values 10 18o values on a cm scale in the shear zone are consistent with peak temperature and accompanying deformation post dating at least in part fluid infiltration the low permeability and lack of brittle overprinting of the shear zone are inconsistent with fluid flow post dating deformation modelling of the inferred oxygen isotope exchange demonstrates that fluid rock interaction must have occurred at either high temperatures for infiltrating water with a 18o value of 10 w r is 1 at 700 c or with very low 18o values for the infiltrating fluids if the infiltrating fluid had 18o values 250 c for exchange to occur in a reasonable time period even if the 18o values were abnormally low previous studies have suggested fluid infiltration into mid crustal fault systems and shear zones may play an important role in the rheological evolution of these structures this study provides direct evidence for such infiltration and suggests that the identification of similar settings may help to establish the frequency and significance of these processes

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

a stable isotope study of microbial dolomite formation in the coorong region south australia abstract the coorong region of south australia is a classic location for the study of modern dolomite formation a number of ephemeral lakes here contain dolomite whilst other lakes in close proximity contain aragonite with minor magnesite our previous field and experimental work has revealed that sulphate reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area we here use a combination of oxygen carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments we report isotopic data from four dolomite and three aragonite containing lakes 34 s data for dissolved lake and porewater sulphates reveal 34 s enrichment of residual sulphate in all seven lakes consistent with bulk closed system microbial fractionation by sulphate reducing bacteria calculations using temperature dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes a comparison of 18 o data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt like saturated mud horizon coincident with the zone of sulphate reduction surprisingly 13 c data from the carbonates do not show the characteristic wide range of values often associated with organogenic carbonates but instead show that these bacterially mediated precipitates incorporate carbon primarily from the inorganic lakewater reservoir partially diluted by an organic component a coorong type microbial model of dolomite formation may be particularly applicable to for example precambrian sedimentary dolomites where benthic microbial communities which are today restricted to extreme environments dominated the shallow marine ecosystem prior to the emergence of metazoa

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

petrographic and geochemical evidence for the formation of primary bacterially induced lacustrine dolomite la roda white earth pliocene central spain upper pliocene dolomites white earth from la roda spain offer a good opportunity to evaluate the process of dolomite formation in lakes the relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations the la roda mg carbonates dolomite unit occur as a 3 5 to 4 m thick package of poorly indurated white massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds the massive dolomite beds consist mainly of loosely packed 1 to 2 m sized aggregates of dolomite crystals exhibiting poorly developed faces which usually results in a subrounded morphology of the crystals minute rhombs of dolomite are sparse within the aggregates both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites in addition aggregates of euhedral dolomite crystals are usually present in some more clayey sepiolite interbeds the thin porous carbonate mostly dolomite beds exhibit both euhedral and subrounded bacterially induced dolomite crystals the carbonate is mainly ca dolomite 51 54 mol caco3 showing a low degree of ordering degree of ordering ranges from 0 27 to 0 48 calcite is present as a subordinate mineral in some samples sr mn and fe contents show very low correlation coefficients with mg ca ratios whereas sio2 and k contents are highly correlated 18o and 13c values in dolomites range from 3 07 to 5 40 pdb mean 0 06 1 75 and from 6 34 to 0 39 pdb mean 3 55 1 33 respectively samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18o and 13c between the two minerals the correlation coefficient between 18o and 13c for dolomite is extremely low and negative r 0 05 whereas it is higher and positive r 0 47 for calcite the lacustrine dolomite deposit from la roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow hydrologically closed perennial lake the lake was supplied by highly saturated hco3 co32 groundwater that leached dolomitic mesozoic formations precipitation of dolomite from alkaline lake waters took place under a semi arid to arid climate however according to our isotopic data strong evaporative conditions were not required for the formation of the la roda dolomite a significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence

upper permian vertebrates and their sedimentological context in the south urals russia fossil fishes and tetrapods amphibians and reptiles have been discovered at 81 localities in the upper permian of the southern urals area of european russia the first sites were found in the 1940s and subsequent surveys have revealed many more broad scale stratigraphic schemes have been published but full documentation of the rich tetrapod faunas has not been presented before the area of richest deposits covers some 900 000 km2 of territory between samara on the river volga in the nw and orenburg and sakmara in the sw a continental succession some 3 km thick of mudstones siltstones and sandstones deposited on mudflats and in small rivers flowing off the ural mountain chain span the last two stages of the permian kazanian tatarian the succession is divided into seven successive units of kazanian kalinovskaya osinovskaya and belebey svitas in succession and tatarian age which is further subdivided into the early tatatian urzhumian gorizont bolshekinelskaya and amanakskaya svitas in succession and the late tatarian severodvinian vyazovskaya and malokinelskaya svitas of equivalent age and vyatkian gorizonts kulchumovskaya and kutulukskaya svitas of equivalent age this succession documents major climatic changes with increasing aridity through the late permian the climate changes are manifested in changing sedimentation and the spread of dryland plants and peak aridity was achieved right at the permo triassic ptr boundary coincident with global warming uplift of the urals and extinction of land plants led to stripping of soils and massive run off from the mountains these phenomena have been identified at the ptr boundary elsewhere south africa australia and this may be a key part of the end permian mass extinction the succession of late permian fish and tetrapod faunas in russia documents their richness and diversity before the mass extinction the terminal permian kulchomovskaya and kutulukskaya svitas have yielded respectively some 6 and 13 species of fishes sharks bony fishes lungfishes and 11 and 14 species of tetrapods aquatic amphibians herbivorous and carnivorous reptiles of all sizes up to the hippo sized pareiasaurs and sabre toothed gorgonopsians immediately following the end permian environmental catastrophe earliest triassic faunas consisted only of a few fish taxa and small aquatic tetrapods in low diversity low abundance assemblages

large volume low δ18o rhyolites of the central snake river plain idaho usa the miocene bruneau jarbidge and adjacent volcanic fields of the central snake river plain southwest idaho are dominated by high temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3 samples from units representing at least 50 of this volume are strongly depleted in 18o with magmatic feldspar 18ovsmow vienna standard mean ocean water values between 1 4 and 3 8 the magnitude of the 18o depletion and the complete lack of any rhyolites with normal values 7 10 combine to suggest that assimilation or melting of a caldera block altered by near contemporaneous hydrothermal activity is unlikely instead we envisage generation of the high temperature rhyolites by shallow melting of idaho batholith rocks under the influence of the yellowstone hotspot affected by eocene meteoric hydrothermal events the seeming worldwide scarcity of strongly 18o depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths

aerobic microbial dolomite at the nanometer scale implications for the geologic record microbial experiments are the only proven approach to produce experimental dolomite under earth s surface conditions although microbial metabolisms are known to induce dolomite precipitation by favoring dolomite growth kinetics the involvement of microbes in the dolomite nucleation process is poorly understood in particular the nucleation of microbially mediated dolomite remains a matter for investigation because the metabolic diversity involved in this process has not been fully explored herein we demonstrate that halomonas meridiana and virgibacillus marismortui two moderately halophilic aerobic bacteria mediate primary precipitation of dolomite at low temperatures 25 35 c this report emphasizes the biomineralogical implications for dolomite formation at the nanometer scale we describe nucleation of dolomite on nanoglobules in intimate association with the bacterial cell surface a combination of both laboratory culture experiments and natural samples reveals that these nanoglobule structures may be 1 the initial step for dolomite nucleation 2 preserved in the geologic record and 3 used as microbial tracers through time and or as a proxy for ancient microbial dolomite as well as other carbonate minerals

oxygen isotope diversity in the anorogenic koegel fontein complex of south africa a case for basement control and selective melting for the production of low δ18o magmas koegel fontein about 350 km north of cape town is the only known early cretaceous anorogenic igneous complex along the volcanic rifted margin of south africa the oldest rocks of the complex are minor granite and syenite intrusions at 144 ma which were followed by tholeiitic and alkaline basalt dykes then by microsyenite and quartz porphyry dykes the youngest and largest igneous unit is the 135 ma rietpoort granite with an exposed diameter of about 20 km the country rocks are mesoproterozoic gneisses of the namaqua natal province which in many places were deformed and retrogressed by pan african tectonism whole rock 18o values from the rietpoort granite and smaller plutonic units syenite granite are in the range 6 9 outliers 4 and 17 quartz 18o values from all units are in a narrow range and indicate magma 18o values between 6 and 8 in contrast to the syenites and granites most mafic and silicic dyke units have 18o

co2 levels in the late palaeozoic and mesozoic atmosphere from soil carbonate and organic matter satpura basin central india abstract a number of calcic palaeosols have been identified within the fluvial deposits of the motur permian the denwa triassic the bagra jurassic and the lameta cretaceous formations of the satpura sedimentary succession central india these palaeosols show accumulation of pedogenic carbonates in rhizocretions and glaebules the carbon isotopic compositions of these carbonates and the coexisting soil organic matters are used to determine the isotopic composition and the partial pressure of atmospheric co 2 using the co 2 palaeobarometer developed by cerling am j sci 291 1991 377 it is seen that the atmospheric co 2 level increased by a factor of 8 from the permian to the jurassic and declined again during the cretaceous the nature of the changes agrees with the result of the co 2 evolution model of berner geocarb ii but the magnitude of the co 2 increase in the middle jurassic and the late cretaceous was higher than the predicted value degassing of earth s interior due to rapid break up of the gondwana landmass during the triassic and jurassic period could have caused the rapid co 2 increase

overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the madrid miocene basin spain the miocene distal alluvial fan and palustrine deposits of the madrid basin paracuellos de jarama area were examined to establish the sequence of its pedogenic diagenetic processes and the main factors controlling them in this area the diagenetic processes operated not only on carbonates as commonly studied but also in high magnesium clays and opaline cherts this paper provides a dynamic model for saline alkaline lake margins that complements the existing freshwater palustrine model three sections ber pel and upc containing limestones sepiolite dolostones and opaline cherts were examined these represent the entire transition from alluvial fan deposit to lake environment the alluvial fan deposits pel section show the most complex sequence of processes after a weak edaphic imprint the primary mudstone and siltstones are affected by silicification opaline levels formed mainly by selective replacement of sepiolites and later dolomitization several types of dolomite are present rounded crystals spherules and globular bodies being the most characteristic in the ponds situated at the foot of the alluvial fans or in interfan areas sepiolite precipitated within free water bodies ber section and an intense pedogenesis led to the formation of palustrine sepiolite deposits later the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging in the shallow lakes upc section palustrine limestones and sepiolite were deposited the carbonates are partially affected by silicification the sepiolite becoming completely silicified the opaline levels clearly preserve the structure of the limestones and sepiolite host rocks all the opaline levels show local aging processes the silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies in the distal alluvial fan sediments of the pel section the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures the silicification environment was therefore not easy to determine in the ber and upc section the main silicification took place in groundwater influenced environments but evidence was seen of cements and silicifications in vadose environments in all study settings the overlapping of pedogenic vadose and groundwater processes was controlled by changes in the position of the water table four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements the majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin in all the studied environments the sepiolite deposits were preferentially silicified their fibrous structure and high absorption capacity helping to retain interstitial fluids moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments the dissolution of the sepiolite caused an increase in silica in the interstitial fluids favouring silicification and ruling out the need of a biogenic source the replacement of limestones by silica is usually driven by variations in ph and salinity which are very common in surface environments such as shallow lakes and soils

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

microbial precipitation of dolomite in methanogenic groundwater we report low temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments in a freshwater aquifer microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls in the laboratory ordered dolomite formed at near equilibrium conditions from groundwater with molar mg ca ratios of 1 dolomite was absent in sterile experiments geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation we hypothesize that the attached microbial consortium reacts with the basalt surface releasing mg and ca into solution which drives dolomite precipitation via nucleation on the cell wall these findings provide insight into the long standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions

microbial precipitation of pedogenic calcite pedogenic calcite in desert soils has become increasingly important as an indicator of paleoclimate landscape stability and landscape age this study indicates that calcic and petrocalcic horizons in desert soils are not simply the result of inorganic precipitation of calcite soil microorganisms were found to be involved in calcite precipitation in a typical desert soil near las cruces new mexico fossilized remains of calcified fungal hyphae and microcodium structures are abundant in the petrocalcic horizon soil bacteria and fungi precipitated calcite when cultured on a ca rich medium in an experiment where soil columns were irrigated with ca rich solutions calcite formed in soils containing soil microorganisms but no calcite formed in sterile soils thus biomineralization of calcite by soil microorganisms appears to be an important mechanism of unknown magnitude

a low δ 18 o intrusive breccia from koegel fontein south africa remobilisation of basement that was hydrothermally altered during global glaciation abstract the cretaceous koegel fontein igneous complex is situated on the west coast of south africa and has a high proportion of rocks with abnormally low 18o values the rocks with the lowest 18o values 5 2 belong to intrusive matrix supported breccia pipes and dykes containing a variety of clast types the breccia rocks range in sio2 from 44 to 68 wt and their whole rock 18o values vary between 5 2 and 1 8 the major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts based on the nature of the clasts contained in the breccia it was emplaced just prior to intrusion of the main rietpoort granite at 134 ma all components of the breccia have low 18o value and at least in the case of the gneiss clasts this predates incorporation in the fluidized material although the early cretaceous appears to have been a period of cold climate it is unlikely that the 18o values of ambient precipitation 10 would have been low enough to have generated the required 18o depletion the basement gneiss was probably 2 3 km below the cretaceous surface minimizing the possibility of interaction with isotopically unmodified meteoric water and there is no evidence for foundered blocks of cover rocks in the breccia there is therefore no evidence for downwards movement of material we favour a model where basement gneiss interacted with extremely 18o depleted fluid during crustal reworking at 547 ma a time of global glaciation low 18o metamorphic fluids produced by dehydration melting of 18o depleted gneiss became trapped and as the fluid pressure increased failure of the seal resulted in explosive upwards movement of fluidized breccia migration was along pre existing dykes incorporating fragments of these dykes as well as the country rock gneiss

late permian continental sediments in the se iberian ranges eastern spain petrological and mineralogical characteristics and palaeoenvironmental significance abstract a detailed mineralogical and petrological study and the analysis of paleosol profiles in continental alluvial sediments of the late permian in the se iberian ranges spain allow us to infer the significant environmental changes that occurred during this time period three parts have been distinguished in the late permian sediments alcotas formation the lower part includes abundant and well preserved carbonate paleosol profiles and fine grained sediments made up by quartz feldspar hematite and illite with scarce kaolinite the preservation of dolomicrite in some paleosols suggests that they originally developed as dolocretes in an arid to semi arid climate with marked seasonality a change towards more humid and acid conditions can be deduced from the presence of siderite and goethite in paleosols in the middle part of the alcotas formation moreover the presence of plant remains coal beds and or carbonaceous shales at the top of the middle part and the lack of carbonate paleosols in the upper part of the formation would indicate a further step towards acid conditions these conditions would increase until the early triassic as indicated by the lack of carbonates and the presence of sr rich aluminium phosphate sulphates aps minerals at the base of the triassic canizar formation which clearly indicates extreme acid conditions during the permian triassic transition of the study area

pedogenic phreatic carbonates on a middle devonian givetian terrigenous alluvial deltaic plain gilwood member watt mountain formation northcentral alberta canada in the muskeg trough of northcentral alberta the gilwood member contains widespread carbonate deposits that formed within terrigenous mudstone and sandstone hosts stratigraphic depositional and petrographic relationships indicate that these carbonates represent calcretes and dolocretes calcretes observed best with cathodoluminescence display microcrystalline alpha fabrics circumgranular cracks root networks displacive growth fabrics elongate channel voids and rare coloform growths with flower spar similarly dolocretes have microcrystalline alpha fabrics brecciation gradational contacts with host mudstones extensive layered nodular horizons and are associated with anhydrite and pyrite 13c values range between 7 to 1 and 6 to 3 for calcretes and dolocretes respectively oxygen isotopes are more variable and differ with host lithologies 18o of calcretes ranges between 11 to 8 for sandstones and 8 to 3 for mudstones whereas 18o of dolocretes ranges between 3 to 1 for marine mudstones and 6 to 2 for pedogenic mudstones regional mapping indicates that calcretes thicken towards the deepest parts of the muskeg trough widespread dolocretes extend beyond the eastern and western limits of muskeg trough and are useful marker intervals for regional correlations dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcretized channel sandstones calcrete isotopic values are interpreted as indicative of carbonate precipitation from waters with meteoric water input however the higher 18o values in dolocretes are indicative of a contribution from an isotopically heavier source such as seawater stratigraphically calcretes are most common along the western and northern edges of muskeg trough thus calcrete accumulation was further controlled by meteoric water in flow from the highland to the west and sluggish groundwater flow in muskeg trough in contrast regionally widespread dolocrete horizons appear to have formed from mixing of fresh waters derived from the highland to the west and seawaters introduced from the east regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out flow from the permeable channel sandstones which resulted in calcretization in channel proximal mudstones and dolomitization in channel distal mudstones

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

biomagnetostratigraphy of the vikinghogda formation svalbard arctic norway and the geomagnetic polarity timescale for the lower triassic a bio magnetostratigraphy for the lower triassic is constructed using the ammonoid biostratigraphy from arctic boreal successions combined thermal and alternating field demagnetisation determines the triassic magnetic field polarity in 86 of specimens with 36 showing linear trajectory line fits and the remainder great circle trends towards the characteristic magnetisation mean pole directions for the deltadalen 50 159 dp dm 3 9 5 1 lusitaniadalen 56 163 dp dm 4 4 5 4 and vendomdalen 57 143 dp dm 4 4 5 4 members fall close to the european lower triassic apparent polar wander path mean directions for two of these member means pass the reversal test the remanence is predominantly carried by magnetite the polarity stratigraphy when integrated with the ammonoid and meager conodont data is similar to that determined from successions in the sverdrup basin canada the permian triassic boundary post dates a pronounced palynofloral turnover and pre dates a short duration reverse magnetozone lt1n 1r in the correlated shangsi section in s china lt1n 1r occurs after the fad of h parvus but in the arctic is within the otoceras boreale zone the late griesbachian to early smithian is mostly reverse polarity with three normal polarity intervals overlain by mid and late smithian normal polarity the spathian contains four reverse polarity intervals the oldest one within the early spathian with the remainder in the late spathian transition into the anisian is within the uppermost reverse magnetozone a feature documented in other sections of this age the polarity pattern is correlated to other marine sections indicating the robustness of the bio magnetostratigraphic composite and its utility in calibrating lower triassic time

evaluation of models for the origin of miocene low δ18o rhyolites of the yellowstone columbia river large igneous province two models have been proposed for the origin of the low 18o rhyolites of the snake river plain yellowstone hotspot trend the first involves a single multicyclic caldera system or multiple nested or overlapping systems in which the early erupted products become hydrothermally altered and are subsequently melted to form low 18o rhyolites bindeman and valley 2001 watts et al 2011 the second model appeals to a crustal magma source which was hydrothermally altered in a thermal event prior to the onset of miocene magmatism boroughs et al 2005 we critically evaluate the plausibility of each and provide thermal temporal and volume constraints which must be included in any model for the petrogenesis of low 18o rhyolites of the central snake river plain although the models are not mutually exclusive we conclude that a component of pre existing low 18o crust is required in the central snake river plain where the greatest volumes of strongly 18o depleted rhyolite occur this component could be provided by meteoric hydrothermally altered granites of the idaho batholith which are cut by the central snake river plain

assessing pedogenic calcite stable isotope values can positive linear covariant trends be used to quantify palaeo evaporation rates abstract palaeoclimate models suggest enhanced evaporation rates in subtropical regions 15 30 latitude during greenhouse world conditions however there are no empirical data to support these estimates laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend plct in 18 o calcite versus 13 c calcite values the goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo evaporation rates from palaeosol calcrete plcts samples of powdered caco 3 were dissolved in de ionized water in pressure sealed containers with a range of elevated atmospheric p co 2 concentrations for 24 h the solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted and aliquots removed for analysis every 24 h the precipitated calcite produced an array of 18 o calcite versus 13 c calcite values that fall upon a plct projected from a theoretical meteoric calcite line mcl water analyses yielded 18 o water enrichments from an initial value of 4 75 vsmow ranging up to between 10 0 and 14 8 the experimental results show that solutions formed under higher p co 2 conditions precipitated calcite very early on during evaporation and thus have 18 o calcite and 13 c calcite values that are slightly enriched relative to the mcl the solutions that formed under low p co 2 conditions precipitated calcite after much of the fluid had evaporated and thus yield more enriched 18 o calcite and 13 c calcite values repeat trials with varying temperature and relative humidity show that the plct is steeper under both higher temperature and low relative humidity the wide range of p co 2 temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution precipitation reactions the results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to caco 3 while the primary factors affecting the magnitude and steepness of the plct are vadose zone temperature relative humidity the starting 18 o water value and saturation state of the fluid with respect to caco 3 since the p co 2 of the rooting zone is cross controlled by the local soil and surface ecology the magnitude of the plct enrichment may not be a direct proxy for palaeo evaporation rates

an unusual marine crocodyliform from the jurassic cretaceous boundary of patagonia remains of the marine crocodyliform dakosaurus andiniensis from western south america reveal a lineage that drastically deviated from the skull morphology that characterizes marine crocodyliforms the snout and lower jaw are extremely robust short and high and only bear a few large teeth with serrated edges resembling those of some terrestrial carnivorous archosaurs this unusual morphology contrasts with the long and gracile snout and lower jaws bearing numerous teeth which are present in the closest relatives of d andiniensis and interpreted as indicating feeding on small fish or mollusks thus the morphological diversity of pelagic marine crocodyliforms was wider than had been thought

stable isotope composition of tropical high altitude fresh waters on mt kenya equatorial east africa abstract the stable isotope composition of equatorial high altitude precipitation and surface water bodies has been studied on mt kenya 0 10 s 37 20 e in east africa during july 1997 regarding the local hydrology glacier meltwater was identified as the main source of replenishment for the lakes while direct precipitation and runoff appear to have a negligible contribution during this period of the year two through flow lakes exhibit nearly no evaporative isotopic enrichment relative to the inflow isotopic composition while two longer residence time lakes display an oxygen and hydrogen isotopic build up of 4 0 7 0 and 18 0 34 0 respectively the data indicate that stream waters are composed of a mixture of high altitude glacier meltwater and rainfall the isotopic composition of an ice sample obtained directly from the margin of a glacier is enriched relative to the average composition of the glacier suggesting mixing with local rain samples of ground frost crystals have an isotopic composition similar to the rain or slightly enriched in addition the results show the lack of a significant altitude effect on the mountain this study demonstrates the importance of the characterization of local hydrological settings when interpreting lacustrine equatorial isotopic records of past climate and confirms that for east africa the total range in oxygen and hydrogen isotopic composition of meteoric waters is smaller than in high latitudes

aspects of ultrabasic and basic alkaline intrusive rocks from bitterfontein south africa cretaceous alkaline intrusive rocks present in the bitterfontein area comprise a suite having compositions ranging from olivine melilitite to syenite these rocks which include some of lamprophyric affinity form part of the widespread dominantly alkaline suite of intrusives occurring in a broad zone parallel to the south western african coastal regions this paper presents the petrology of the bitterfontein alkaline rocks together with mineral and rock analyses the chemistry of the rocks shows the suite to define a sio2 enrichment trend linking the compositions of olivine melilitite to oversaturated basic rocks this trend is compared with trends defined by essentially tholeiitic volcanics and kimberlite olivine melilitite compositions and a genetic relationship with the latter is apparent polybaric crystallisation of a larnite normative liquid in a crustal environment coupled with fractionation especially of phlogopite is important in the generation of members of the bitterfontein suite phlogopite fractionation represents a deviation by the bitterfontein suite from the evolutionary trend predicted by experimental work for low pressure anhydrous compositions in the fo la ne sio2 system

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

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

palaeoenvironmental significance of late permian palaeosols in the south eastern iberian ranges spain the late permian wuchiapingian alcotas formation in the se iberian ranges consists of one red alluvial succession where abundant soil profiles developed detailed petrographical and sedimentological studies in seven sections of the alcotas formation allow six different types of palaeosols with distinctive characteristics and different palaeogeographical distribution to be distinguished throughout the south eastern iberian basin these characteristics are in turn related to topographic climatic and tectonic controls the vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the late permian some few millions of years before the well known permian triassic global crisis

sedimentology and geochemistry of carbonates from lacustrine sequences in the madrid basin central spain abstract lacustrine and alluvial carbonate facies have been investigated in middle miocene successions of the western side of the madrid basin in order to evaluate paleoenvironments in which carbonates formed carbonate facies are varied and include 1 calcrete and dolocrete 2 pond deposits 3 lake margin dolostone 4 mudflat carbonate and 5 open lake carbonate facies the dominant mineralogy of these is dolomite and or low mg calcite no high mg calcite or aragonite have been detected in any sample 18 o and 13 c values range from 8 20 to 1 80 pdb and 10 25 to 0 70 pbd respectively more negative 18 o and 13 c values correspond to predominantly calcite calcretes and to carbonate deposited in ponds at the foot of arkosic alluvium higher 18 o values are from both lacustrine carbonate and dolocrete this latter lithofacies has strong geochemical similarities to dolostones deposited in a lake margin environment mudflat carbonate deposited on shallow platforms subject to lake water oscillation shows great heterogeneity in both stable isotope value and trace element content the mineralogy of these carbonates is dominated by calcite and the limestones contain molds of gypsum occurrence of calcitized dolomite textures in these facies suggests the influence of fresher water during expanding lacustrine cycles or further interaction with less saline groundwater trace element contents are considered to be potential indicators of the different carbonate facies types thus aiding the paleoenvironmental interpretation however discrimination among carbonate facies on the basis of trace element contents appears to be dependent on the statistical method utilized for treatment of data more information is needed to ascertain their use as paleoenvironmental indicators

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

δ13c depth profiles from paleosols across the permian triassic boundary evidence for methane release stable carbon isotopic analyses of organic carbon 13 c in individual paleosol profiles from permian triassic sequences of antarctica reveal systematic isotopic variations with profile depth these variations are in many cases analogous to those in modern soils which are functions of redox conditions soil development and degree and type of microbial decay in modern soils these isotopic depth functions develop independently from vegetation changes c 3 versus c 4 vegetation and can be diagnostic of soil orders this study shows that soil intrinsic functions can be preserved in the 13 c values of paleosols as old as 260 ma and constitute valuable data for paleoecological interpretations a large carbon isotopic offset of as much as 10 in whole paleosol profiles across the permian triassic boundary indicates significant changes in the soil biogeochemistry and the soil atmosphere system early triassic paleosols are distinctive in their extremely low 13 c values to 42 and often show an anomalous 13 c depth distribution compared to both permian paleosols and modern soils highly depleted 13 c values as the ones in early triassic paleosols are suggested to be associated with microbial methane oxidation methanotrophy this hypothesis implies increased methane concentrations in the early triassic soil atmosphere system increased atmospheric methane was probably partly responsible for the global carbon isotopic shift documented in marine and terrestrial sediments across the permian triassic boundary

early triassic aggradational and degradational landscapes of the karoo basin and evidence for climate oscillation following the p tr event the lower triassic katberg formation beaufort group karoo supergroup in south africa is a fine grained arenaceous unit deposited under ephemeral bedload dominated fluvial conditions and previously interpreted as representing a continuous stratigraphic record its genesis was linked directly to the end paleozoic extinction event and was considered to exhibit the results of a trend towards aridification the present study documents the lowermost katberg formation exposed at carlton heights northern cape province to test the aridification hypothesis four lithofacies vary in both vertical and lateral relationships with feldspathic sandstone dominating the stratigraphy pisolith size nodular conglomerate consists of pedogenically derived carbonate nodules originally precipitated within aridisols none of which are either exposed or reported from the formation rather the only evidence of extreme aridification is found within channel lag barform and overbank deposits marking landscape degradation landscape aggradation is characterized by two feldspathic sandstone geometries where 1 medium bedded planar bedded or ripple laminated sheet sandstone is interpreted as ephemeral sheetflood deposits and 2 thick bedded cross laminated multi lateral lenticular sandstone is interpreted as associated with deeper sand bed braided anabranching systems overbank fines consist of polyphase siltstone paleosols in which bioturbated inceptisols were overprinted by gleysols wherein large pedogenic carbonate nodules precipitated under a high regional water table evidence exists for periodic drying in these wet regimes but no evidence exists for extreme aridity hence the lowermost katberg formation superficially exhibits only an aggradational sedimentological record but significant diastems exist as evidenced by the concentration of aridisol precipitates at the base of each degradational cycle a model for the early triassic in this part of the karoo basin is presented wherein this record is interpreted as a function of climate oscillations rather than either episodic uplift in the cape fold belt or any ecosystem response to the end permian extinction

dryland continental mudstones deciphering environmental changes in problematic mudstones from the upper triassic carnian to norian mercia mudstone group south west britain

oxygen isotopes in indian plate eclogites kaghan valley pakistan negative δ18o values from a high latitude protolith reset by himalayan metamorphism abstract oxygen isotope compositions are reported for the first time for the himalayan metabasites of the kaghan valley pakistan in this study the highest metamorphic grades are recorded in the north of the valley near the india asia collision boundary in the form of high pressure hp group i and ultrahigh pressure uhp group ii eclogites the rocks show a step wise decrease in grade from the uhp to hp eclogites and amphibolites the protoliths of these metabasites were the permian panjal trap basalts ca 267 2 4 ma which were emplaced along the northern margin of india when it was part of gondwana after the break up of gondwana india drifted northward subducted beneath asia and underwent uhp metamorphism during the eocene ca 45 1 2 ma at the regional scale amphibolites group i and ii eclogites yielded 18o values of 5 84 and 5 91 1 66 to 4 24 and 2 25 to 0 76 respectively relative to vsmow on a more local scale within a single eclogite body the 18o values were the lowest 2 25 to 1 44 in the central the best preserved least retrograded parts and show a systematic increase outward into more retrograded rocks reaching up to 0 12 these values are significantly lower than the typical mantle values for basalts of 5 7 0 3 the unusually low or negative 18o values in group ii eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the indian plate was at southern high latitudes 60 s the stepwise increase in 18o values among different eclogite bodies in general and at single outcrop scales in particular reflects differing degrees of resetting of the oxygen isotope compositions during exhumation related retrogression

methane release from igneous intrusion of coal during late permian extinction events abstract unusually large and locally variable carbon isotope excursions coincident with mass extinctions at the end of the permian period 253 ma and guadalupian epoch 260 ma can be attributed to methane outbursts to the atmosphere methane has isotopic values 13c low enough to reduce to feasible amounts the carbon required for isotopic mass balance the duration of the carbon isotopic excursions and inferred methane releases are here constrained to 10 000 yr by counting annual varves in lake deposits and by estimating peat accumulation rates on paleogeographic maps the most marked carbon isotope excursions form linear arrays back to plausible methane sources end permian siberian traps and longwood bluff intrusions of new zealand and end guadalupian emeishan traps of china intrusion of coal seams by feeder dikes to flood basalts could create successive thermogenic methane outbursts of the observed timing and magnitude but these are unreasonably short times for replenishment of marine or permaf

stable isotope geochemistry and formation mechanisms of quartz veins extreme paleoaltitudes of the central alps in the neogene quartzveins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the central swiss alps in some cases the veins are completely filled with milky quartz while in others sometimes spectacular void filling quartz crystals are found the style of vein filling and size is controlled by host rock composition and deformation history temperatures of vein formation estimated using stable isotope thermometry and mineral equilibria cover a range of 450 c down to 150 c vein formation started at 18 to 20 ma and continued for over 10 my the oxygen isotope values of quartz veins range from 10 to 20 permil and in almost all cases are equal to those of the hosting lithology the strongly rock buffered veins imply a low fluid rock ratio and minimal fluid flow in order to explain massive nearly monomineralic quartz formation without exceptionally large fluid fluxes a mechanism of differential pressure and silica diffusion combined with pressure solution is proposed for early vein formation fluid inclusions and hydrous minerals in late formed veins have extremely low d values consistent with meteoric water infiltration the change from rock buffered static fluid to infiltration from above can be explained in terms of changes in the large scale deformation style occurring between 20 and 15 ma the rapid cooling of the central alps identified in previous studies may be explained in part by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more an average water flux of 0 15 cm3 cm 2yr 1 entering the rock and reemerging heated by 40 c is sufficient to cool rock at 10 km depth by 100 c in 5 million years the very negative d values of

carbon isotope fractionation during decomposition of organic matter in soils and paleosols implications for paleoecological interpretations of paleosols abstract stable isotope ratios reported as 13c of organic carbon preserved in paleosols have potential utility as a valuable paleoecological indicator if the isotopic composition of paleobiomass is preserved in the paleosol organic carbon without fractionation however soil organic matter is in a constant state of decomposition in an open system and frequently decomposes further during alteration after burial of the paleosol during this decomposition kinetic fractionation of 13c 12c in an open system results in concentration of 13c in the residual organic matter preserved in the soil and paleosol this paper summarizes studies of modern soils which show that this 13c enrichment follows a rayleigh distillation process whereby the 13c 12c ratio is a function of the natural logarithm of the fraction of remaining soil organic carbon leaving soil organic carbon 13c enriched by up to 6 with respect to the original biomass if the fraction of original biomass carbon remaining during these decomposition processes deviates significantly from 1 the soil organic carbon can be 13c enriched by several per mil and thus not a good paleoecological indicator of the 13c value of the original biomass therefore effective paleoecological studies of 13c preserved in paleosol organic carbon must therefore demonstrate that the measured concentration of organic carbon in the paleosol is near its maximum concentration during soil formation or that the paleosol organic matter does not include the 13c enriched products of microbial decomposition suggestions are made for interpreting the effects of carbon isotope fractionation of organic matter and for materials and methods most useful for paleoecological studies from bulk paleosol organic carbon

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

petrogenesis of low δ 18 o quartz porphyry dykes koegel fontein complex south africa this paper investigates the origin of low 18o quartz porphyry dykes associated with the 144 133 ma koegel fontein igneous complex which was intruded during the initial phase of breakup of africa and south america the 25 km diameter rietpoort granite is the largest and youngest phase of activity and is roofed by a 10 km diameter pendant of gneiss quartz porphyry qp dykes up to 15 m in width strike nw se across the complex the qp dykes that intruded outside the granite have similar quartz phenocryst 18o values average 8 0 0 7 n 33 to the granite average 8 3 1 0 n 7 the qp dykes that intruded the roof pendant have quartz phenocrysts with more variable 18o values average 1 6 2 1 n 55 in some cases quartz phenocrysts have 18o values as low as 2 5 the variation in 18o value within the quartz crystal population of individual dykes is small relative to the overall range and core and rim material from individual quartz phenocrysts in three samples are identical within error there is no evidence that quartz phenocryst 18o values have been affected by fluid rock interaction based on a quartz magma value of 0 6 magma 18o values must have been as low as 3 1 samples collected along the length of the two main qp dykes that traverse the roof pendant have quartz phenocryst 18o values that range from 1 1 to 4 6 and 2 3 to 5 6 respectively these 18o values correlate negatively r 0 96 with initial 87sr 86sr which can be explained by the event that lowered 18o values of the source being older than the dykes we suggest that the qp dykes were fed by magma produced by partial melting of gneiss which had been variably altered at high temperature by 18o depleted meteoric water during global glaciation at 550 ma the early melts had variable 18o value but as melt pockets interconnected during melting the 18o values approached that of average gneiss variable quartz phenocryst 18o values in the same dyke can be explained by vertical emplacement at variable rates of ascent along the dyke the lateral variation in quartz and hence magma 18o value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke and the age of the particular magma batch

abrupt and gradual extinction among late permian land vertebrates in the karoo basin south africa the karoo basin of south africa exposes a succession of upper permian to lower triassic terrestrial strata containing abundant terrestrial vertebrate fossils paleomagnetic magnetostratigraphic and carbon isotope data allow sections to be correlated across the basin with this stratigraphy the vertebrate fossil data show a gradual extinction in the upper permian punctuated by an enhanced extinction pulse at the permian triassic boundary interval particularly among the dicynodont therapsids coinciding with negative carbon isotope anomalies

zircon oxygen isotopic constraint on the sources of late mesozoic a type granites in eastern china abstract a type granites are usually derived from partial melting of the lower crust and or the upper mantle within extensional settings but their exact sources for specific plutons are required to unravel a systematic laser fluorination analysis of oxygen isotopes was carried out for five well documented late mesozoic a type granites in eastern china zircon from 41 samples was separated and 109 aliquots of fresh zircon were analyzed along with whole rock powder and quartz from the same samples hydrothermally altered whole rock samples show scattered 18 o values of 2 3 to 7 4 with a total variation up to 9 7 in contrast zircon 18 o values vary only from 3 1 to 5 4 with a range of 2 3 most available zircon 18 o values of a type granites are lower than normal mantle zircon values of 5 3 0 6 2sd and high temperature equilibrium fractionations are well preserved between quartz and zircon in most cases the above lines of evidence suggest that the low 18 o zircons in a type granites were crystallized from low 18 o magmas because low 18 o magmas cannot be derived entirely from the mantle reworking and or recycling of crustal igneous rocks are involved for their petrogenesis the low 18 o zircons in this study thus offer evidence for a dominantly crustal origin of the late mesozoic a type granites in eastern china

geology petrology and o and h isotope geochemistry of remarkably 18o depleted paleoproterozoic rocks of the belomorian belt karelia russia attributed to global glaciation 2 4 ga abstract this paper deals with strongly 18o depleted down to 27 3 vsmow 1 9ga paleoproterozoic mid grade metamorphic rocks found in the belomorian belt of karelia e baltic shield the protolith of these rocks is attributed to have been altered by glacial meltwaters during the world s first 2 4 2 3 ga paleoproterozoic slushball glaciation when karelia was located near equatorial latitudes we describe in detail three and report seven new localities with unusually depleted 18o signatures that now span 220 km across the belomorian belt hydrogen isotope ratios measured in amphibole biotite and staurolite also display remarkably low values of 212 to 235 isotope mapping in the three best exposed localities has allowed us to identify the world s most 18o depleted rock located at khitostrov with a 18o value 27 in khitostrov samples zircons have normal 18o detrital cores and low 18o metamorphic rims mapping demonstrates that zones of 18o depletion occur in a concentric pattern 100 400 m in dimension and each locality displays significant 18o and d heterogeneity on a meter to centimeter scale characteristic of meteoric hydrothermal systems worldwide the zone of maximum 18o depletions usually has the highest concentration of metamorphic corundum rutile and zircon and also display doubled concentrations of insoluble trace elements zr ti cr hree these results are explained by elemental enrichment upon mass loss during hydrothermal dissolution in ph neutral meteoric fluid remarkably low 18o and d values suggest that alteration could have only happened by glacial meltwaters in a subglacial rift zone many localities with 18o depletions occur inside metamorphozed 2 4 ga gabbro noritic intrusions or near their contact with belomorian gneisses implying that the intrusions were driving meteoric hydrothermal systems during the known 2 4 ga episode of belomorian rifting given that the isotopically depleted localities now spread over 200 km the extent of the karelian ice cap is estimated to be at least that large svecofennian 1 9 ga metamorphism is seen to cause metamorphic recrystallization of hydrothermally altered rocks into coarse grained assemblages and causing local metasomatism through devolatilization of the underlying hydrous low 18o protolith further depleting d via volatilization this process led to gem quality rubies and kyanites that preserve these remarkable 18o values in the geologic record

o isotope study of the bushveld complex granites and granophyres constraints on source composition and assimilation the 2 05 ma bushveld magmatic event culminated in the production of490 000 km 3 of granite and granophyres in these granitic rocks high temperature equilibrium o isotope fractionations are generally preserved between quartz and zircon but not between quartz and feldspar or between biotite and amphibole quartz separated from four granite samples shows no significant difference in core and rim d 18 o values which indicates that quartz is not significantly zoned and provides further evidence that it is unaffected by alteration quartz can therefore be used as a proxy for the magma d 18 o value leading to estimates of 6 9o for both the granites assuming quartz magma 1 11o and granophyres assuming quartz magma 0 62o similar magma d 18 o values 6 6o were obtained using zircon d 18 o values assuming zircon magma 1 3o the initial nd isotope ratio of the granitic rocks ranges from 0 509676 to 0 509822 with an average value of 0 509655 n 12 this corresponds to average end values of 5 9 and 4 8 for the granites and granophyres respectively the similarity in isotope composition between the granites and granophyres and between the granitic rocks from each of the three major lobes of the bushveld complex is consistent with a common origin the d 18 o values of the granitic rocks suggest derivation from mantle derived magmas by either fractional crystallization or partial melting but this hypothesis is incompatible with their crustal end values average 5 5 the associated rustenburg layered suite rls rocks have average d 18 o values of 7 1o which is within error of the average estimate for the bushveld granitic rocks and similar end values however granitic magma derived from the same paretal magmas that produced the rls would have had an average magma d 18 o of about 7 9o 1 o higher than observed we therefore suggest that the granitic magmas were produced by fractional crystallization of rls magma or by partial melting of solidified rls magma at depth followed by assimilation at a shallower level of a significant quantity of hydrothermally altered low d 18 o material from the since eroded volcanic edifice

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

voluminous low δ18o magmas in the late miocene heise volcanic field idaho implications for the fate of yellowstone hotspot calderas we report oxygen isotope compositions of phenocrysts and u pb ages of zircons in four large caldera forming ignimbrites and post caldera lavas of the heise volcanic field a nested caldera complex in the snake river plain that preceded volcanism in yellowstone early eruption of three normal 18 o voluminous ignimbrites with 18 o quartz 6 4 and 18 o zircon 4 8 started at heise at 6 6 ma and was followed by a 2 3 18 o depletion in the subsequent 4 45 ma kilgore caldera cycle that includes the 1800 km 3 kilgore ignimbrite and post kilgore intracaldera lavas with 18 o quartz 4 3 and 18 o zircon 1 5 the kilgore ignimbrite represents the largest known low 18 o magma in the snake river plain and worldwide the post kilgore low 18 o volcanism likely represents the waning stages of silicic magmatism at heise prior to the reinitiation of normal 18 o silicic volcanism 100 km to the northeast at yellowstone the occurrence of low 18 o magmas at heise and yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex sudden shifts in 18 o of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low 18 o signature from mantle or crustal sources instead 18 o age trends indicate progressive remelting of low 18 o hydrothermally altered intracaldera rocks of previous eruptions this trend may be generally applicable to older caldera complexes in the snake river plain that are poorly exposed

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

hydrochemical characteristics of aquifers near sutherland in the western karoo south africa the western karoo depends entirely on groundwater for domestic and agricultural use an understanding of the chemical evolution of the groundwater provides insight into the interaction of water with the environment and contributes to better resource management descriptive statistics correlation matrices and factor analysis together with stable isotope data were used to gain an understanding of the hydrochemical processes of the groundwaters in the fractured rocks around sutherland in the western karoo the main processes influencing the groundwater chemistry are salinisation mineral precipitation and dissolution cation exchange and human activity the stable isotope data indicate that the infiltration of evaporated water is partly responsible for the occurrence of saline subsurface waters the ionic oxygen and hydrogen stable isotope composition of the groundwaters suggests that the effects of localised topography are one of the overriding factors controlling the groundwater chemistry in higher lying areas ca hco3 2 type waters are prevalent while in topographical flat areas nacl type waters dominate in areas where water collects close to or at the surface saline soils are formed from these areas salts are leached to the subsurface during significant recharge periods these findings are important in that they contribute to the identification of suitable locations for developing groundwater resources that could yield a relatively better quality water q 2001 elsevier science b v all rights reserved

sudden changes in fluvial style across the permian triassic boundary in the eastern iberian ranges spain analysis of possible causes abstract the sedimentary record of the late permian and early triassic of the eastern iberian ranges shows four major sudden or very rapid vertical changes in fluvial style the late permian sedimentary cycle starts with the boniches formation of alluvial fan braided fluvial origin which grades vertically over within a few metres into the alcotas formation deposited by low to high sinuosity avulsion prone rivers with extensive floodplains the alcotas formation contains calcimorphic soils plant remains and pollen and spore assemblages however the upper third of the unit is devoid of all organic remains and soils and is characterized by a dominant red colour the sandstone levels were deposited by high sinuosity meandering rivers this major change took place during the late permian and is probably coeval with the emplacement of the emeishan basaltic large igneous province lip in se china rocks of the boniches and alcotas formations are separated by an angular unconformity from the overlying strata which consist of the late permian conglomeratic hoz del gallo formation of alluvial fan gravel braided fluvial origin and the sandy canizar formation of low sinuosity sandy river origin the permian triassic boundary lies probably between the upper part of the hoz del gallo formation and the first metres of the canizar formation late permian pollen and spore assemblages have been found in the hoz del gallo formation but the canizar formation is barren with the exception of an anisian middle triassic assemblage at the top tectonic extensional pulses in the iberian basin caused the changes observed between the lower and upper parts of the boniches formation at the base of the hoz del gallo formation and between the lower and upper part of this formation the changes observed in the uppermost part of the alcotas formation are not easily explained by tectonic causes nor those in the passage from the hoz del gallo formation to the canizar formation similar sedimentary characteristics of the sandy canizar formation such as amalgamated sandstone bodies erosion and reactivation surfaces dominant trough cross stratification tabular geometry absence of plant remains and pollen and spores and absence of silts and clays to those of coeval formations in places as far away as australia south africa and brazil suggest a global rather than local cause for these abrupt changes in fluvial style this global cause was probably die off of plant cover over extensive areas of the catchment related to the end of the permian mass extinction and possibly related to the emplacement of the west siberian basaltic large igneous province lip responsible for drastic atmospheric and marine changes

oxygen and neodymium isotope evidence for source diversity in cretaceous anorogenic granites from namibia and implications for a type granite genesis abstract many of the early cretaceous intrusive complexes in the damaraland of west central namibia are silicic in composition although all have trace element characteristics typical for the so called a type granites major differences in alkali aluminum ratios and isotopic compositions require diverse magma sources this paper presents nd and o isotope data from the five largest silicic complexes paresis erongo brandberg cape cross gross spitzkoppe that provide new constraints on the nature of crustal and mantle sources involved and their relative proportions the paresis complex has an isotopic signature 18o 9 end130 ma 21 indicating a crustal component similar to mesoproterozoic gneisses of the angola craton the other complexes have isotope variations 18o from 8 1 to 10 7 and end130 ma from 1 to 9 that can be explained by a binary mixing model between a mantle and crustal component more importantly this same mixing line also fits the nd o isotope variations reported from the mafic okenyenya and messum complexes and from rhyodacites in the southern etendeka volcanic sequence the uniformity of the crustal component implied by this mixing model suggests lower crustal material in contrast to the geologic complexity of the neoproterozoic damara belt presently exposed at the surface this is consistent with the isotopic data and we interpret the crustal component to be lower crustal metametasediments that were dehydrated and perhaps melt depleted by generation of the s type granites which are widespread in the damara belt the mantle component is interpreted to be dominated by the tristan mantle plume but some involvement of depleted mantle material is needed to explain all of the isotope data the data rule out any significant role for enriched subcontinental mantle lithosphere all silicic damaraland complexes as well as the etendeka rhyodacites classify as a type granites despite their proven source diversity this means that the a type geochemical characteristics of these and by analogy other anorogenic granites does not depend on a particular magma source the unifying feature of the diverse granites in the damaraland is likely to be related to the high temperatures involved which allow partial melting of dehydrated and possibly melt depleted lower crustal lithologies

causes and consequences of extreme permo triassic warming to globally equable climate and relation to the permo triassic extinction and recovery permian waning of the low latitude alleghenian variscan hercynian orogenesis led to a long collisional orogeny gap that cut down the availability of chemically weatherable fresh silicate rock resulting in a high co2 atmosphere and global warming the correspondingly reduced delivery of nutrients to the biosphere caused further increases in co2 and warming melting of polar ice curtailed sinking of o2 and nutrient rich cold brines while pole to equator thermal gradients weakened wind shear and associated wind driven upwelling lessened further diminishing productivity and carbon burial as the earth warmed dry climates expanded to mid latitudes causing latitudinal expansion of the ferrel circulation cell at the expense of the polar cell increased coastal evaporation generated o2 and nutrient deficient warm saline bottom water wsbw and delivered it to a weakly circulating deep ocean warm deep currents delivered ever more heat to high latitudes until polar sinking of cold water was replaced by upwelling wsbw with the loss of polar sinking the ocean was rapidly filled with wsbw that became increasingly anoxic and finally euxinic by the end of the permian rapid incursion of wsbw could have produced 20 m of thermal expansion of the oceans generating the well documented marine transgression that flooded embayments in dry hot pangaean mid latitudes the flooding further increased wsbw production and anoxia and brought that anoxic water onto the shelves release of co2 from the siberian traps and methane from clathrates below the warming ocean bottom sharply enhanced the already strong greenhouse increasingly frequent and powerful cyclonic storms mined upwelling high latitude heat and released it to the atmosphere that heat trapped by overlying clouds of its own making suggests complete breakdown of the dry polar cell resulting rapid and intense polar warming caused or contributed to extinction of the remaining latest permian coal forests that could not migrate any farther poleward because of light limitations loss of water stored by the forests led to aquifer drainage adding another 5 m to the transgression non peat forming vegetation survived at the newly moist poles climate feedback from the coal forest extinction further intensified warmth contributing to delayed biotic recovery that generally did not begin until mid triassic but appears to have resumed first at high latitudes late in the early triassic current quantitative models fail to generate high latitude warmth and so do not produce the chain of events we outline in this paper future quantitative modeling addressing factors such as polar cloudiness increased poleward heat transport by deep water and its upwelling by cyclonic storms and sustainable mid latitude sinking of warm brines to promote anoxia warming and thermal expansion of deep water may more closely simulate conditions indicated by geological and paleontological data

reworked calcretes their significance in the reconstruction of alluvial sequences permian and triassic minorca balearic islands spain the permian and triassic of minorca balearic islands consists of a 670 m thick red alluvial succession that includes in situ calcrete profiles and reworked calcrete material in the permian succession the calcretes vary from laminar forms developed on the carboniferous basement to weakly developed nodular calcretes in fluvial sediments the palaeosols in the triassic are mostly dolomitic and the profiles reach up to stage iii of soil development spec pap geol surv am 203 1995 1 the clasts formed through reworking of the palaeosol profiles are about 0 5 10 cm across and include mosaics of calcite dolomite crystals brecciated clasts rhizolith fragments and aggregates of clay and or silt these clasts appear in three different types of deposits type 1 corresponds to lenticular bodies that fill small scour surfaces and consists only of intraformational conglomerates these deposits are interpreted as ephemeral channels and sheet floods that represent the interfluvial drainage systems that captured only the precipitation falling on the alluvial plain type 2 includes sand dune 3 d bodies with flat bottoms and convex tops these bodies are about 20 cm high and 2 m wide and were formed by floodwaters that flowed down the levees of the major streams type 3 channel deposits contain reworked calcretes and extrabasinal clasts which overlie erosive surfaces and are found in layers within cross bedded sandstones and conglomerates these are interpreted as channel floor lag deposits of major channels that entered from distant uplands and drained the alluvial plain variations in the aggradation rates of the floodplain resulted in five different infill stages in the lowstand to early transgressive interval as in stages i p1 and iv b1 the fluvial deposits filled palaeovalleys calcretes and reworked calcrete deposits were of difficult formation apart from terraces and preservation accommodation space was at its greatest in the transgressive stages ii p2 and v b2 this caused the greatest aggradation of the floodplains which are formed of thick sequences of fine grained sediments isolated meandering channels weakly developed calcretes compound and reworked calcrete deposits mostly of types 1 and 2 the density of channels notably increased in stage iii p3 highstand interval because of the reduction of accommodation space this could favour the formation of composite or even cumulative palaeosols but of difficult preservation reworked calcrete deposits are mostly of type 3 but types 1 and 2 are also recognised the reworked calcrete deposits are an important part of the permian and triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins and the construction of floodplains

vertebrate extinction across permian triassic boundary in karoo basin south africa distinct assemblages of paleosols above and below the permian triassic boundary in the karoo basin of south africa are evidence for reorganization of ecosystems following this greatest of all mass extinctions the permian triassic boundary is recognized from the last appearance of dicynodon and from a series of negative excursions in the isotopic composition of carbon within therapsid tusks pedogenic carbonate nodules and organic matter the boundary is also marked by laminated beds with very weakly developed paleosols a change from purple 10r to brownish red 2 5yr paleosols and a thin 10 cm claystone breccia of reworked soil clasts paleoclimatic changes include a shift from arid and highly seasonal paleoclimate inferred from diffuse and shallow calcareous nodules in permian paleosols to semiarid and less seasonal paleoclimate inferred from deep and well focused calcic horizons in triassic paleosols an earliest triassic shift to warmer and wetter paleoclimate is also indicated by increased chemical weathering abundance of lycopsids and diversity of labyrinthodonts permian paleosols have root traces comparable to those of open shrubland and riparian woodland whereas triassic paleosols have root traces and profile forms like soils of open woodland this is a significant paleoenvironmental change but not as dramatic a change as would be expected from the devastating extinctions of 88 of fossil vertebrate genera latest permian therapsid reptiles were diverse and ecologically specialized in contrast the principal earliest triassic therapsid lystrosaurus was a burrower with no specific habitat preference its short internal nares barrel chest and high neural spines would have given it greater aerobic scope than preexisting therapsids and may have been an advantage under conditions of hypercapnia and hypoxia these adaptations and associated ecosystem changes are compatible with widespread vertebrate mortality by acidosis and pulmonary edema in a post apocalyptic greenhouse created by the voluminous release of methane from shallow marine and permafrost clathrates

low δ 18 o mantle derived magma in panjal traps overprinted by hydrothermal alteration and himalayan uhp metamorphism revealed by sims zircon analysis abstract we report two generations of low 18o zircons from the himalayan eclogites and their host gneisses in situ sims 18o analyses on single zircon crystals with known age and hf isotope ratios from two populations of chemically distinct zircons demonstrate a complex history 1 an early low 18o mantle derived magma 2 followed by post emplacement high temperature meteoric water alteration and finally 3 crystallization of new low 18o minerals during the ultrahigh pressure metamorphism magmatic zircon 269 ma in group i eclogites yielded 18o values from 1 9 to 4 6 vsmow with an average value of 4 0 0 2 n 35 the error is 2sd analytical precision and n represents number of analyzed spots which is lower than the typical mantle values 5 3 0 6 2sd in contrast metamorphic zircons 45 ma in group ii eclogites preserve unusually low negative 18o values from 3 9 to 2 7 average 3 4 0 4 n 35 2sd zircons in felsic gneiss that surround group ii eclogites have inherited magmatic cores ca 260 ma with 18o values of ca 2 9 which decrease to 0 1 in metamorphic ca 45 ma rims these zircons preserve lower 18o values than would be equilibrated with typical mantle the low 18o values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative 18o values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons this study reports evidence for melting of subducted low 18o ocean crust to form low 18o mantle derived mafic magmas as previously proposed by cartwright and valley 1991 geology 19 578 581 for proterozoic scourie dikes

pedogenic carbonates as a proxy for palaeo co2 in the palaeozoic atmosphere abstract according to a model by cerling cerling t e 1991 carbon dioxide in the paleoatmosphere evidence from cenozoic and mesozoic paleosols am j sci 291 377 400 cerling t e 1999 stable carbon isotopes in palaeosol carbonates in thiry m simon coincon r eds palaeoweathering palaeosurfaces and related continental deposits vol 27 of spec publ int assoc sediment pp 43 60 the carbon isotope composition of calcretes should depend on the soil type and the co 2 concentration in the atmosphere we have tested cerling s model by investigating 14 palaeozoic sections with soil profiles a large number of carbonate types of different genetic origin exist in the localities examined comparing the palaeozoic samples with recent and subrecent calcretes it can be demonstrated that anhedral cryptocrystalline 2 concentration

groundwater dolocretes from the upper triassic of the paris basin france a case study of an arid continental diagenetic facies thick dolomite cemented horizons dolocretes occur within a fluvial sandstone mudstone sequence of late triassic age in the western part of the paris basin france two types of dolomites can be distinguished a nodular dolomitic beds less than a few metres thick which formed within mottled overbank siltstones and mudstones and b massive dolomite up to 16 m thick which occurs in coarse grained channel sandstones and conglomerates the majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and less commonly dolomicrite glaebules irregular spar filled cracks spheroidal dolomite silicification and vuggy porosity are locally abundant in the massive dolomite in contrast biologically induced micromorphological features such as rhizocretions and alveolar septal fabrics were observed in the thin nodular dolomite beds the dolomite is near stoichiometric well ordered and non ferroan 18o values range from 7 7 to 0 4 o pdb and 18o values range from 5 1 to 1 8 0 pdb and no obvious difference in the stable isotopic composition between both types of dolomites was observed sr isotope ratios range from 0 7101 to 0 7126 and are invariably higher than the contemporary triassic sea water a vadose pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures several features however argue against a pedogenic origin for the massive carbonates most notably the absence of biologically induced structures the occurrence in coarse grained channel and not overbank deposits and the great thickness these units are thus interpreted as groundwater in origin phreatic calcretes of quaternary age widespread in inland australia are regarded as a modern analogue for the triassic paris basin dolocretes petrographic observations argue in favour of primary proto dolomite precipitation although early diagenetic replacement of calcite by proto dolomite cannot be ruled out strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non marine continental origin for the groundwaters the poorly ordered and non stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near stoichiometric well ordered dolomite that clearly lacks evidence for pervasive recrystallization

the origin and significance of pedogenic dolomite from the upper permian of the south urals of russia pedogenic carbonate nodules from six sections spanning the continental permian triassic boundary in the south urals russia were analysed morphological petrographic sem and xrd analyses have demonstrated that many of the latest permian palaeosols are dolomitic this dolomite forms the microcrystalline 5 16 m groundmass of the nodules later diagenetic phases represented by coarser crystalline textures were identified as calcite isotopic analysis of the microcrystalline dolomite has revealed it to be similar in isotopic composition to authigenic dolomite forming today in saline soils in alberta canada these data indicate that the dolomite found in these nodules is pedogenic and formed in equilibrium with the atmosphere upper permian pedogenic dolocretes in the studied sections are most frequent in a palaeosols that formed on palaeo highs and b in the latest permian period changhsingian which may indicate that there was an increase in seasonality and evaporation in the south urals region at this time the presence of only calcitic palaeosols in the earliest triassic may reflect a subsequent dramatic change in the basin conditions possibly relating to the permian triassic mass extinction which stopped the conditions that are necessary for dolomite formation

δ13c values of carbonate nodules across the permian triassic boundary in the karoo supergroup south africa reflect a stinking sulfurous swamp not atmospheric co2 abstract paleosol macromorphology petrography and stable isotope geochemistry of carbonate nodules from permo triassic p t boundary strata of the central karoo basin south africa are presented in order to evaluate their utility as geochemical archives of permian triassic paleoatmospheric chemistry and their potential to provide a terrestrial carbon isotope chemostratigraphy for correlation with contemporaneous global marine 13 c records paleosol morphologies across p t boundary strata of the central karoo basin indicate the region was a poorly drained seasonally to continuously flooded bottomland radiaxial calcite microspar and micrite microtextures are observed in the calcite nodules associated with p t paleosol profiles micritic calcite is typical of soil carbonate that forms in well drained soils characterized by open system gaseous diffusion between the soil and global atmosphere whereas more coarsely crystalline textures and radiaxial calcite are more typical of crystallization in phreatic environments with limited gaseous diffusion between the soil and global atmosphere in this regard only those nodules composed of micritic calcite can be considered as a potentially reliable source of proxy information for paleoatmospheric 13 c values across the p t boundary the 13 c values of all carbonate nodules from two overlapping measured sections near carlton heights south africa range from 24 4 to 1 8 whereas micritic calcite nodule 13 c values range from 7 0 to 21 3 comparison of calcite 13 c values with penecontemporaneous organic matter 13 c values indicate that calcite 13 c values 13 c values rather these more negative carbonate nodule 13 c values likely reflect calcite crystallization under poorly drained swampy conditions characterized by a semi closed chemical system that evolved independently of earth s troposphere therefore previously reported negative carbon isotope shift s measured from carbonate nodules in the karoo basin p t strata do not provide a record of atmospheric 13 c and probably do not provide a viable carbon isotope stratigraphy that can be realistically correlated with global p t marine carbon isotope stratigraphies