role of corrosion by h2so4 fallout in cave development in a travertine deposit evidence from sulfur and oxygen isotopes abstract 18o and 34s values for spring water dissolved sulfates and sulfides air borne sulfur species and gypsum found on the walls of two caves in the travertine deposit of the cave and basin hot springs in banff national park alberta canada suggest that the caves are developing at least in part via a chemical corrosion process microbiological reduction of dissolved sulfate with positive 18o and 34s values produces reduced sulfur species hs h2s with negative 34s values upon escape from the spring water the h2s is converted to h2so4 fallout h2so4 attacks the calcite in the travertine leading to the formation of gypsum the gypsum on the walls of the caves has negative 34s values similar to those of the reduced sulfur species and negative 18o values that reflect incorporation of an appreciable proportion of oxygen derived from spring water vapor during h2s oxidation hydrolysis isotopically similar gypsum in spring pools at the site represents gypsum that has fallen off the walls exposing fresh travertine to attack by sulfuric acid

sedimentology of quaternary perched springline and paludal tufas criteria for recognition with examples from guadalajara province spain outcrop scale geometries and bed relationships of ambient temperature freshwater carbonates are poorly understood because many described tufas have been dismantled by erosion and present only part of a particular depositional model at the field scale four end member models encompass the tufa continuum 1 perched springline 2 paludal 3 fluvial and 4 lacustrine tufas individual bed types can occur with variable dominance within several of these models but one or more beds are characteristically dominant only within a single tufa model so it can be differentiated from relatively isolated outcrop fragments two models perched springline tufas and paludal tufas are known in outline only in the literature despite being present within the quaternary deposits of most karstic regions perched springline tufas generally form lobate fan shaped mound morphologies on hillslopes and develop from single or multiple spring resurgences mature deposits show a subhorizontal top and a steep face on the downflow side the steep outer zones of tufa mounds may be developed into cascades with moss curtains or can be dominated by shallow rimstone pools according to face angle tufa deposits lying downslope of the mounds are usually detrital in nature especially if some dismantling of the mound has occurred the relatively thin subhorizontal lobe top deposits commonly contain organic rich deposits paludal tufas develop predominantly in waterlogged valley bottom situations where line sourced waters emerge from valley side and bottom aquifers lime mud precipitation predominates in these sites mud is deposited as subhorizontal laminites that thin towards the valley axis and downstream of resurgences tufa spring mounds may form where lesser volumes of water are involved individual tussocks phytoherm cushions of grasses and rushes are the most diagnostic feature of the model but sapropels and peats may be intercalated diagenesis in both models is rapid lithification of individual beds is virtually instantaneous and always occurs before the decay of the associated living vegetation the resulting highly porous and permeable fabrics remain fresh in holocene tufas but aggrading neomorphism and partial spar infill of vegetation moulds is common in older deposits dissolution in many perched springline tufas is small scale many large cavities are primary but with later coatings of speleothems early removal of organics from paludal tufas is responsible for autobrecciation and differential compaction

comment and reply on weathering before the advent of land plants evidence from detrital k feldspars in cambrian ordovician arenites

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

cryogenic carbonates in cave environments a review abstract cryogenic cave carbonate ccc represents a specific type of speleothem its precipitation proceeds at the freezing point and is triggered by freezing induced concentration of solutes compared to classical speleothems stalagmites flowstones ccc occurs as accumulations of loose uncemented aggregates the grain sizes range from less than 1 m to over 1 cm in diameter karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology rapid freezing of water results in the formation of ccc powders with grain size typically below 50 m slow freezing of water in caves usually in systems where the co2 escape is partly restricted e g ice covered water pools results in the formation of large mineral grains with sizes from less than 1 mm to about 20 mm the range of carbon and oxygen stable isotope compositions of ccc is larger than for a typical carbonate speleothem rapid freezing of water accompanied by a quick kinetic co2 degassing results in large ranges of 13c of the ccc powders between 10 and 18 pdb slow freezing of water with a restricted co2 escape results in gradual increase of 13c values from 9 to 6 pdb data ranges in individual caves are usually much more restricted accompanied by a 18o decrease of the precipitated carbonate overall range from 10 to 24 pdb these unusual trends of the carbonate 18o evolution reflect incorporation of the heavier 18o isotope into the formed ice new isotope data on ccc from three romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water cccs are proposed as a new genetic group of speleothems

the middle cambrian mount roosevelt formation new of northeastern british columbia evidence for rifting and development of the kechika graben system the eastern margin of the kechika graben in the vicinity of muncho and moose lakes northeastern british columbia contains middle cambrian rift related strata herein named the mount roosevelt formation the formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late early cambrian gataga group bonnia olenellus zone and is conformably overlain by a thick unnamed carbonate sequence of middle cambrian age plagiura poliella zone the mount roosevelt formation is subdivided into three members the lowermost member is characterized by ooid bearing siltstone and sandstone interbedded with dolostone limestone and hematitic conglomerate conformably above this the middle member is a thick sequence of polymict cobbly pebble conglomerate the upper member includes karstified dolostone calcareous cemented conglomerate and sandstone and limestone collectively the mount roosevelt formation reflects alluvial fan delta progradation into a transgressive mar

ordovician alluvial fan to marine shelf transition in sw sardinia western mediterranean sea tectonically sardic phase influenced clastic sedimentation abstract a localized lower ordovician early caledonian structural event sardic phase is recorded in sw sardinia by an angular and erosional unconformity faulting and mild folding this event was accompanied by sedimentation of a transgressive clastic unit puddinga beds which is particularly well developed along a major fault system gonnesa fault zone there the puddinga beds range from basal interfingering fans some formed by coarse carbonate and schist clasts others by schist clasts only to fluvio lacustrine mud and gravelly sandstone to coastal muddy and sandy deposits and to fossiliferous restricted marine shelf sediments the existence of the slightly compressive sardic phase in sw sardinia and quasi coeval calc alkaline volcanism and an extensional structural phase sarrabese phase in central east sardinia suggest transcurrent faulting the calc alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position the non volcanic sequence of sw sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the gondwanaland craton this sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from nw spain structural activity has been the predominant control on sedimentation however worldwide recurring low sea level stands may have influenced the sedimentation of the lower part of the puddinga beds and the deglaciation of the saharan ice sheet has contributed to the uppermost ordovician early silurian deepening of the euxinic shelf

co2 release variations during the last 2000 years at the colli albani volcano roma italy from speleothems studies abstract the colli albani is the quiescent volcano that dominates the southwestern skyline of roma italy the last eruption occurred during the holocene from the eccentric albano maar along its western slope the volcano is presently affected by cyclic seismic swarms ground uplift and diffuse co 2 degassing the degassing has caused several deadly incidents during the last years and constitutes a major civil protection concern as the volcano slopes are densely inhabited nevertheless the volcano does not have a permanent monitoring network and the background level and anomalous co 2 levels the relationship between the gas release and the seismic and ground deformation activity at the colli albani are still to be defined the aim of this work is to define the historical record of co 2 release evidences of deep co 2 periodic release during the last 2000 years in the area of colli albani volcano roma italy are offered from speleothems studies a roman age stone mine now used for mushroom cultivation is decorated with actively growing speleothems characterised by depositional hiatuses different levels of four stalactites separated by depositional unconformities and several samples from a single depositional cycle belonging to a stalagmite have been dated by u th method and analysed for their o and c isotopic composition eight cycles of deposition have been identified from 90 110 a d to 1350 1370 a d some of which are recognised across different speleothems the age gap dividing different growth layers is in the order of one to few hundred years giving a temporal span for periodic interruption of speleothems deposition o and c isotopic analyses performed on the samples collected from a single cycle the oldest have shown that the composition of the mother solutions was initially mainly meteoric and that a progressive increase in the input of a deep component rich in co 2 up to a proportion of 20 30 occurred just before the interruption of the speleothem deposition this could be due to a progressive increase of the acidity of the water solutions that caused the undersaturation of fluids if we extrapolate this mechanism to the other cycles of deposition being characterised by analogue isotopic compositions we can hypothesise periods of deposition interrupted by episodes of co 2 release which in the colli albani volcano are often recorded in coincidence with earthquakes therefore we have correlated the hiatuses with some of the largest historical earthquakes interesting to the city of rome

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

carbonate diagenesis in ice the fine particles created by glacial comminution of carbonate minerals are susceptible to interaction with meltwater and the isotopic composition of fine carbonate in glacial deposits has the potential of recording this interaction here we focus on the carbonate fraction of glacial sediment and debris within basal ice at the tsanfleuron glacier switzerland size separates of basal ice debris and melt out tills show a collinear isotopic trend interpreted as a mixing line between detritus 18 o pdb 5 and 13 o pdb 0 6 and authigenic calcite which in one case had 18 o values as light as 18 7 and 13 c values as light as 5 7 in most cases the finer fractions with calcite crystals as small as 0 1 m are enriched in the isotopically light component which is interpreted to form largely by freezing in basal ice the isotopic compositions of ice and included gases could be influenced by this process melt out tills inherit the authigenic calcite thus carbonate bearing till should no longer be regarded simply as clastic sediment

palaeoproterozoic evaporites in fennoscandia implications for seawater sulphate the rise of atmospheric oxygen and local amplification of the δ13c excursion this paper addresses global oxygenation and establishment of a marine sulphate reservoir in the palaeoproterozoic we report syn depositional marine anhydrite containing pseudomorphs after ca sulphates as widespread throughout the tulomozero formation in the se fennoscandian shield implying that surface waters were oxidized and a large so42 marine reservoir was developed as early as 2100 ma the ca sulphates and associated magnesite and halite precipitated syn depositionally from oxidized evolved and modified seawater in coastal playa sabkha and intertidal flat settings sr 87 sr 86 and delta 13 c of associated c 13 rich stromatolitic dolostones were environmentally controlled with the highest ratios occurring in playa and sabkha carbonates the results imply that the palaeoproterozoic delta 13 c carb excursion was amplified by 8 by local environmental factors and calls into question many observations of putative delta 13 c global signals reported previously from similar palaeoproterozoic evaporitic dolostones the local environmental amplification can explain a large regional and intercontinental delta 13 c discrepancy observed in synchronous carbonates

effect of the advent and diversification of vascular land plants on mineral weathering through geologic time the origin of vascular land plants in the silurian and their subsequent diversification have had a major effect on mineral weathering through geologic tune the presence of vascular plants reduces the stability of soil minerals through a net export of ions from soil waters and through the release of complexing organic acids by root mycorrhizae additional factors that dictate the nature of plant induced mineral weathering are 1 the differences in nutrient dynamics between evergreen and deciduous species 2 the role of specific nutrient sinks biomass storage and secondary soil mineralization and outputs runoff etc in plant ecosystems and 3 the effect of long and short term ecosystem disturbances first order increases in overall mineral weathering probably took place in the middle paleozoic and early tertiary following the initial colonization and diversification of land plants and the radiation of deciduous angiospenns second order fluctuations would typify time intervals where paleoecosystem disturbances were maximized such as periods of climatic instability

cryogenic cave calcite from several central european caves age carbon and oxygen isotopes and a genetic model abstract cryogenic cave calcite ccc formed by segregation of solutes during water freezing was found in three central european caves this calcite type forms accumulations of loose calcite grains on cave floor the calcite grains are of highly variable crystal morphology and of sizes ranging from less than 1 mm to over 1 cm the most typical feature is their accumulation as loose uncemented crystals u series dating indicates the formation of ccc in the studied caves during several climatic oscillations of the weichselian between 61 and 36 ka bp in the chelsiowa jama jaskinia jaworznicka cave system in poland between 34 and 26 ka bp in the buml cave in the czech republic and between 26 and 21 ka bp in the stratenska jasky a cave system slovakia at the time of ccc formation the studied caves were lying in a periglacial zone detailed c and o stable isotope study of ccc samples revealed that slow water freezing under isotope equilibrium was the dominant formational process in the studied polish and czech caves significantly higher 13 c values of ccc in the stratenska jasky a cave indicate either water freezing in a more opened system with continuous co 2 escape rayleigh fractional separation or participation of another co 2 source the model of slow water freezing under isotope equilibrium is supported by isolated character of the caves having limited ventilation in contrast modern cryogenic cave calcite powders sampled directly on the ice surface of two recently iced caves in slovakia with high ventilation showed much higher 18 o and 13 c data similar to cryogenic calcites obtained in experimental rapid water freezing

kalahari valley calcretes their nature origins and environmental significance abstract calcretes that form in non pedogenic settings have been widely reported in the geomorphological and geological literature yet they are still poorly understood in comparison with pedogenic varieties this may be because there are assorted types of non pedogenic calcretes often loosely referred to as groundwater calcretes but encompassing groundwater phreatic open valley or confined channel calcrete types forming within vadose and phreatic environments in different geomorphological settings relatively few studies have described the detailed petrological characteristics of such calcretes leading wright and tucker calcretes international association of sedimentologists reprint series 1991 vol 2 blackwell scientific publications oxford p 10 to suggest that much more work is needed to define the groundwater calcrete facies and to devise criteria for its recognition so that the various non pedogenic calcretes can be identified within the quaternary as well as further back in the geological record most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls thus the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types as a step towards expanding our knowledge of the variety of non pedogenic calcretes this paper describes the petrology micromorphology and mode of development of one of these types of calcrete namely that which forms within valley locations but not in definite fluvial channels late quaternary to holocene valley calcretes are described from various dry valleys mekgacha across the kalahari region of central botswana with the majority of samples collected from trunk and tributary valleys of the okwa samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology in general most samples consist of grains of quartzose kalahari sand cemented by fine crystalline often glaebular grain coating and pore filling micrite cement types are fairly consistent and not as micromorphologically variable as has been noted for other kalahari calcretes biological inputs are prevalent in many samples and include networks of calcified rootlets these characteristics reflect the fact that the calcretes formed in a relatively near surface environment with relatively high rates of evaporation

secondary precipitates in pleistocene and present cryogenic environments mendoza precordillera argentina transbaikalia siberia and seymour island antarctica secondary precipitates iron oxide calcite etc are currently observed in cold climate pleistocene deposits some have micro and ultrastructures quite different from precipitates of vadose phreatic and biogenic origin and seem to have originated by freezing processes the microstructures of calcite coatings from a pleistocene cryopediment in the mendoza pre cordillera from a lower pleistocene cryogenic slope deposit in western transbaikalia and from the present active layer in antarctica are described they show similar patterns fibrous crystals often consisting of piles of platelets some with internal holes assembled in millimetre scale fringes on the lower face of clasts observational mainly fabric evidence confirms that such peculiar crystals are formed during freezing the features are unknown in other climates and when found in past sediments can be diagnostic of cryogenic palaeoenvironments

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

the problem of deep carbon an archean paradox black carbon rich chert dikes are common features of early archean terranes and are especially prominent on the pilbara craton of western australia the dikes cut across the local stratigraphy before truncating abruptly at specific stratigraphic horizons where they can be seen to interfinger with the stratigraphic succession these uniquely archean dike complexes are enigmatic and raise significant questions about the source of the carbon they contain is it biogenic and therefore indicative of the early biosphere or abiotic and perhaps indicative of prebiotic conditions that led to the appearance of the first life on earth the strelley pool chert which is exposed on the pilbara craton of western australia forms part of the warrawoona group and rests unconformably on older ultramafic volcanics the formation which is generally less than 30 m thick consists of a relatively consistent lithologic succession found over a wide area locally a basal transgressive sandstone is overlain by a sequence of carbonate barite black and white laminated chert and grey green chert which is locally stained blue green by weathered fuchsite the formation is intersected by normal faults about which the depositional successions thickens and thins the mineralogy and geochemistry including stable isotope data of the underlying dike complex closely parallels that of the lithologies preserved in the strelley pool chert similarly the depositional sequence of minerals in voids within the dike complex parallels that of the depositional sequence above this new data from the strelley pool chert and its associated dike complex show that the black chert dikes are an integral part of a hydrothermal mineralogical complex the complex accumulated around vents fed by growth fault controlled dikes that extended to a depth of more than 2 km below the archean paleo seafloor the mineralogy of the sedimentary succession reflects deposition during the declining temperature regime of an evolving hydrothermal system driven by an intruding granitoid complex below the fault system and associated hydrothermal complex evolved in the tensional environment developed during doming above the intermittently intruding granitoid complex that forms the framework of the pilbara protocontinent carbonaceous compounds contained within the black chert dikes could have been generated abiotically by a process similar to fischer tropsch synthesis at elevated temperatures 200 300 c and pressures 500 bar at a paleo depth of 2 km within the hydrothermal system the primordial medium in which life first evolved may thus have been a silica rich slurry with a low ph and a high co 2 content which contained basic abiotic organic building blocks fundamental to the origins of life the abiotic organic output from these hydrothermal systems may overwhelm any early biospheric geochemical signal and may therefore be the most difficult environment in which to recognise the early biospheric record 2005 elsevier b v all rights reserved

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

travertine formation in plitvice national park yugoslavia chemical versus biological control hydrochemical studies of the plitvice lakes and their tributaries croatia yugoslavia were coupled with micromorphological investigations on carbonate lake sediments and recent travertines karst springs discharge water from aquifers in triassic and jurassic dolomites and limestones and collect in lakes which are ponded behind accreting travertine dams waters at springs have a high co2 partial pressure greater than 7000 ppm and are slightly undersaturated with respect to calcite saturation index less than 0 03 co2 partial pressure is quickly reduced in swift running streams leading to very high supersaturation with carbonate minerals saturation indices between 0 74 and 0 53 calcite deposition however is restricted to the lake bottoms formation of lake marl and to the tufa dams the annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t caco3 as cascade deposits tufa dams or as micrite in lakes behind the travertine dams the initial stages of travertine formation as a result of morphological biological and chemical factors are i moss settling on small ridges in the creek courses ii epiphytes diatoms and cyanobacteria settling on the moss surface iii micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms and iv inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid carbohydrates come from structural polysaccharides of diatoms the sticky excretions rich in aspartic acid are necessary for the initiation of calcite precipitation they may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters the formation of tufa and micrite lake marl appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits

depositional environments of the early proterozoic espanola formation ontario canada the carbonate rich espanola formation forms part of the huronian supergroup deposited in early proterozoic time between about 2 5 and 2 1 ga ago the espanola formation overlies glacigenic diamict

peritidal carbonate platform growth and cyclicity in an early proterozoic foreland basin upper pethei group northwest canada abstract the upper pethei group is a 170 235 m thick largely limestone succession located in the east arm of great slave lake northwest canada strata accumulated in a paleoproterozoic foreland basin during the late stages of marine deposition rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages 1 marginal peritidal complex 2 back barrier lagoon 3 shelf margin 4 subtidal shelf and 5 slope and basin carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae water column precipitation of carbonate mud and generation and transport of nonskeletal grains stromaclasts ooids oncoids peloids and intraclasts the upper pethei platform is interpreted to have evolved through three basic platform geometries peritidal rimmed shelf subtidal rimmed shelf and subtidal open shelf in response to changes in the rate of creation of accommodation space the peritidal complex and rim behaved in a manner consistent with models of phanerozoic reefal rims and alternatively kept up gave up or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation three scales of cycle are recognized 1 overall formation scale upward deepening of the platform 2 laterally traceable decameter scale shallowing upward cycles separated by marine flooding surfaces and 3 laterally discontinuous meter scale shal owing upward peritidal cycles the first two cycle types are interpreted to have been allogenic controlled by changes in eustasy and subsidence rates meter scale cycles are interpreted as mainly autogenic having formed in an aggradational tidal island model the upper petrel platform is similar to other paleoproterozoic foreland basin carbonate platforms important differences however include 1 laterally discontinuous peritidal cycles 2 lack of widespread sand shoals inboard of the shelf rim 3 a thick predominantly carbonate slope and basin succession 4 moderate to low platformal micrite content 5 extensive lagoonal ooid sands and 6 low contents of terrigenous clastics

properties and development of channel calcretes in a mountain catchment tabernas basin southeast spain nonpedogenic channel calcretes of up to 3 5 m thick occur in a number of locations across the tabernas basin in almeria province southeast spain they represent the last major phase of late quaternary calcium carbonate cementation to affect this semiarid drainage basin channel calcretes are situated within the basal parts of sequences of 12 m thick poorly sorted coarse grained and schist dominated fluvial gravels that were deposited within confined bedrock channels channel calcretes occupy the full basal width of the bedrock channels within which they occur and mostly crop out near the mouths of tributary valleys to the main rambla de tabernas drainage system six profiles from five separate channel calcrete outcrops were logged and sampled all samples were analysed in thin section and under scanning electron microscope sem from these analyses the macro and micromorphological properties were identified and the mode of origin of channel calcrete profiles was assessed results indicate that all channel calcretes are massively cemented by calcite and appear uniform at the field profile scale microscale analyses reveal that detrital grains within the calcretes are initially coated by micrite which is overlain by grain coating and pore filling sparite a series of broad trends in cement type and micromorphology are recognised within individual field profiles all profiles exhibit an increasing degree of calcite crystal size in a down profile direction most field profiles predominantly contain micrite in upper parts with an increasing percentage of sparite towards the base some profiles are however dominated by sparite and show increasing crystal size and occurrence of euhedral crystals towards the profile base many profiles also contain evidence of postcalcite cementation diagenesis in the form of dissolution of calcite crystal faces or replacement of the calcite cement by amorphous silica with alteration mostly occurring in the lower parts of profiles the trends within the calcrete fabric appear to have developed in conjunction with a fluctuating water table with the increased crystal size and occurrence of euhedral crystals towards the base of profiles arising from greater duration of wetting in basal zones cementation in some field profiles occurred at depths of up to 12 m within the host sediment well below the zone of capillary rise and pedogenesis and in the absence of significant organic activity as such channel calcretes in the tabernas basin may represent an ideal opportunity to observe the influence of groundwater upon cementation of coarse grained sediments by carbonate

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

a geochemical evaluation of perennial spring activity and associated mineral precipitates at expedition fjord axel heiberg island canadian high arctic abstract two groups of perennial springs are observed in the canadian high arctic at expedition fjord on axel heiberg island at colour peak and gypsum hill saline discharge 1 3 2 5 molal nacl produces a variety of calcite travertine and gypsum rich precipitates saturation index calculations of the spring waters at colour peak suggest co 2 degassing from the waters causes calcite precipitation gypsum precipitation dominates at gypsum hill where spring waters have lower alkalinity and higher so 4 concentrations mineral accumulations form both channel and rimstone pool morphologies as a result of varying slope conditions at colour peak confined flow in steep slope areas develop massive structures in contrast to more friable porous accumulations in areas where waters fan out on shallower slopes these morphological variations lead to corresponding varying apparent rates of mineral precipitation mineral precipitation at gypsum hill is far less notable as a result of lower discharge rates and annual degradation by icing formation microscopic observations and geochemical analyses of the channel precipitates at colour peak reveal alternating light calcite spar and dark anhedral microcrystalline calcite combined with organic matter and non carbonate minerals laminae rimstone pools forming in lower sections of spring discharge are composed of accumulations of large euhedral calcite crystals interbedded with allochthonous inputs high concentration of dissolved solids is responsible for slow travertine precipitation rates which occurs during winter this precipitation is further retarded during summer months by the introduction of crystal growth inhibitors such as fe 3 and deposition of organic matter and soil sediments

a classification of rhizogenic root formed calcretes with examples from the upper jurassic lower cretaceous of spain and upper cretaceous of southern france reply abstract studies of fluvial sequences from the upper jurassic and lower cretaceous of the cameros basin spain and the upper cretaceous of the aquitaine basin france have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification those from spain have formed by the calcification of root mats whereas those from france resulted from the accumulation of calcified root cells microcodium these root related rhizogenic calcretes developed in stages allowing developmental time series chronosequences to be identified contrasting with existing models for calcrete profile growth they provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings the role of root activity in forming some quaternary calcretes is also discussed

a possible mechanism for growth of calcite speleothems without participation of biogenic carbon dioxide abstract using plummer et al s 11 rate equations on the dissolution and deposition of caco3 in h2o co2 systems we have calculated deposition rates of caco3 to stalagmites in caves which are covered by glaciers or bare karst in this case no biogenic co2 from vegetated soil is available and the deposition of caco3 involves only atmospheric co2 the mechanism of deposition proceeds by a temperature effect cold melting waters of about 0 c dissolve caco3 under open system conditions at the surface of the rock when this solution saturated with respect to caco3 flows through the limestone rock its temperature increases by several degrees therefore it becomes supersaturated and caco3 is deposited under open system conditions in the warmer cave maximal growth rates of about 10 3 cm year are possible from the kinetics of the deposition of caco3 from the thin water films present at the surface of stalagmites we are able to estimate the isotopic composition of carbon in the caco3 deposited on the stalagmites to be approximately 13c 8 which is close to some observed values from our data we conclude that substantial growth of stalagmites is possible during glacial periods as well as in areas of bare karst a question which was not resolved up to now

naturally precipitating vaterite μ caco3 spheres unusual carbonates formed in an extreme environment vaterite a rare hexagonal caco3 polymorph was identified in precipitates forming at a supra glacial sulfur spring in borup fiord pass northern ellesmere island canadian high arctic vaterite occurs in a precipitate mound along with calcite gypsum and native sulfur the unusual conditions of the site including an extremely cold climate supersaturated alkaline waters and the presence of gypsum mimics conditions used to grow vaterite in laboratory experiments stable isotope data suggest that vaterite may preferentially form during colder periods of the year vaterite found at the site is characterized by 2 to 10 m rounded to spherical shaped particles comprising smaller 0 5 to 2 m spheres as both individuals and in chainlike structures the spherical habit of vaterite resembles carbonate structures that have been interpreted to be organic however 13 c values are indicative of an inorganic origin the thick permafrost and the extreme cold and dry environment make this site a good terrestrial analog for carbonate precipitates that might be expected at potential deposits associated with water discharge on other planetary bodies copyright 2003 elsevier science ltd

aragonite laminae in hot water travertine crusts rapolano terme italy small 5 30 m aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at rapolano terme in tuscany italy the aggregates are mainly concentrated in irregular wispy and dark laminae which cross cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit they also occur less commonly scattered irregularly through the calcite layers the aragonite needle aggregates are in the form of crosses fascicles sheaf shaped bundles or dumbbell shaped rosettes and spherulites locally irregular masses of needles also occur the fascicles rosettes and spherulites have hollow centres which resemble microbial components fungal spores bacterial colonies and pollen suggesting that the aragonite crystals are biotically nucleated the lamination is interpreted to reflect diurnal control stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification calcite feather crystals although traversed by the aragonite aggregate laminae have a clear appearance under the light microscope they form more or less continuously through the diurnal cycle by abiotic precipitation the constant association of aragonite with organic nuclei irrespective of whether the latter are in laminae or scattered through the calcite layers supports a biotic control on aragonite formation lamination in pleistocene travertines is superficially similar to that in the present day deposits but is diagenetically altered in the pleistocene deposits the calcite feathers appear dark under the light microscope and the aragonite aggregates where they are not altered to dark calcite are dissolved together with parts of the adjacent spar calcite and therefore appear light coloured

sulfuric acid speleogenesis of carlsbad cavern and its relationship to hydrocarbons delaware basin new mexico and texas sulfur isotope data and ph dependence of the mineral endellite support the hypothesis that carlsbad cavern and other caves in the guadalupe mountains were dissolved primarily by sulfuric acid rather than by carbonic acid floor gypsum deposits up to 10 m thick and native sulfur in the caves are significantly enriched in 32 s isotope 34 s values as low as 25 8 per mil cdt indicate that the cave sulfur and gypsum are the end products of microbial reactions associated with hydrocarbons a model for a genetic connection between hydrocarbons in the basin and caves in the guadalupe mountains is proposed as the guadalupe mountains were uplifted during the late pliocene pleistocene oil and gas moved updip in the basin the gas reacted with sulfate anions derived from dissolution of the castile anhydrite to form h 2 s co 2 and castile limestone the hydrogen sulfide rose into the capitan reef along joints forereef carbonate beds or bell canyon siliciclastic beds and there reacted with oxygenated groundwater to form sulfuric acid and carlsbad cavern a sulfuric acid mode of dissolution may be responsible for large scale porosity of some delaware basin reservoirs and for oil field karst reservoirs in other petroleum basins of the world

genesis of calcrete in quaternary soil profiles hutt and leeman lagoons western australia abstract soil profiles exposed along the margins of hutt and leeman lagoons contain a succession of calcrete horizons that exhibit oolitic pisolitic grainstone lithoclast breccia and pelloidal wackestone mudstone stone lithologies the typical coastal profile consists of an upper laminar calcrete horizon which is usually overlain by a transitional pisolitic loose soil horizon underlying the laminar calcrete is breccia and or massive calcrete downward gradation into the host is through a mottled calcrete horizon structure textures and fabrics are utilized to classify the following different calcrete types pisolitic loose soil is composed of sandy soils calcrete ooids and pisolites eolianite clasts and shell fragments laminar calcrete is composed of parallel laminated sheets of micritic carbonate that develop by segregation processes massive calcrete is dense relatively structureless calcrete with a secondary mudstone fabric resulting from diagenetic alteration of primary constituents calcrete mottles form in response to localized diagenesis in the host sediment breccia calcrete develops as a result of prolonged weathering involving repeated episodes of calcretization dissolution and brecciation diminution of permeability of the host sediment by development of massive calcrete is a major process in genesis of the profile and occurs in the lowermost part of the zone of alternate wetting and drying where the effect of water input overcomes the cumulative effects of evapotranspiration profile development is thereafter topographically controlled and restricted to horizons overlying the massive calcrete redevelopment of zones of alternate wetting and drying in older deposits may produce multiple soil profiles

origin and diagenesis of quaternary travertine shrub fabrics rapolano terme central italy millimetre to centimetre sized arborescent shrub like calcite precipitates are common constituents of hot water travertine shallow pool deposits of quaternary age at rapolano terme tuscany italy in the presently forming travertines the shrubs consist of apparently random associations of i micrite aggregates and ii subhedral to euhedral rhombic spar crystal aggregates in thin section the micrite aggregates appear dark and the spar rhomb aggregates light giving the shrubs a mottled appearance travertines are basically produced by caco3 precipitation due to degassing and evaporation of the spring waters although biological influence may also stimulate precipitation the formation of masses of erect shrubs rather than dense crystal crusts that form on slopes is probably due to limited water flow in the pool environments microbes including bacteria and diatoms are important influences on shrub microfabric and external shape the micrite aggregates are associated with bacteriform bodies seen as tiny rods and spheres the micrite precipitates around these bodies and in adjacent biofilm spar rhomb precipitation appears to be external to the biofilm and may be related to the presence of diatoms which are locally closely associated with the spar rhombs although an essentially inorganic origin particularly for the more euhedral rhombs cannot be ruled out in the older quaternary travertines the original microfabric of the shrubs has been diagenetically altered the original mottled appearance of the shrubs has become uniformly dark and micritic and the evidence for the dual micritic and spar rhomb origin of the shrubs is obscured or destroyed spar micritization of the shrubs is probably due to abiotic and locally biotic dissolution previous studies did not recognize the diagenetic micritization and attributed shrub formation entirely to bacterial activity

the lower permian freshwater carbonates of the slawkow graben southern poland sedimentary facies context and stable isotope study abstract during early permian times the post hercynian rift valley known as the slawkow graben was filled with typical continental molasse deposits including fanglomerates and olistolites playa flat sediments with evaporites volcaniclastics and carbonates deposited in small spring fed basins oasis environment lithofacies variability lithofacies relationships sedimentary structures and paleokarst data indicate a highly changeable sedimentary environment controlled by climate syndepositional faulting and accompanying volcanism differential uplift resulted in a high relief topography the basin margin mountains probably having a relative altitude of up to 1000 m the stable isotopic composition 13 c 18 o of the spring related carbonates indicate seasonal temperature differences of up to 15 c a value that is close to the average annual temperature amplitude in the present day subtropical zone other environmental indicators e g red beds evaporites braided stream sediments corroborate the isotopic interpretation a longer term 6 7 year cycle reflected in 18 o composition of the carbonate is attributed to rainfall fluctuations

groundwater ferricretes from the silurian of ireland and permian of the spanish pyrenees abstract haematite nodule horizons occur in alluvial lacustrine deposits from the mill cove formation silurian of dingle ireland and the unidad roja superior permian of the spanish pyrenees these horizons generally less than 1 m thick are not associated with palaeosol profiles but are hosted by laminated or structureless bioturbated mudstones to sandstones the sequences contain weakly developed calcrete profiles the haematite nodules are interpreted as having formed around fluctuating water tables and are analogous to present day groundwater ferricretes laterites or plinthites fe2 mobilised during periods of reducing groundwaters was segregated into ferric oxide nodules during falls in the water table or during influxes of oxygenated meteoric waters the prevailing climate during deposition of both sequences was probably semi arid and the presence of ferricrete like horizons is not evidence of humid climates the horizons represent early diagenetic effects not related to prevailing soil moisture regime

a stepped karst unconformity as an early silurian rocky shoreline in guizhou province south china abstract where succeeded by marine strata karst unconformities signify a former rocky coastline such relationships may help sort out relative sea level changes and aspects of local geography controlling facies distribution an exceptional example of an early silurian karst shore is well exposed near the village of wudang in central guizhou province near the capital city of guiyang in south china here the lower silurian kaochaitien formation oversteps 63 m of paleotopographic relief in limestones belonging to the llanvirn guniutan formation and caradoc to early ashgill huanghuachong formation ordovician the corresponding rise in sea level took place coeval with tectonic uplift as confirmed by a regionally diachronous relationship in the ordovician silurian boundary across a 250 km track from central to northern guizhou province the change in sea level also fits with a global rise of sea level in late aeronian later llandovery early silurian time borings of the ichnofossil trypanites are reported from the karst surface of the huanghuachong formation and silurian strata fill sink holes in this unit over 5 m deep the silurian karst shoreline near wudang is integrated with other regional data to construct a paleogeographic map covering the northern half of guizhou province

subaerial exposure surfaces in a palaeoproterozoic 13c rich dolostone sequence from the pechenga greenstone belt palaeoenvironmental and isotopic implications for the 2330 2060 ma global isotope excursion of 13c 12c abstract this article reports the discovery of numerous subaerial exposure surfaces which occur in the ca 2200 ma kuetsjarvi sedimentary formation ksf from the pechenga greenstone belt ne fennoscandian shield russia the formation was accumulated within an intraplate rift and is composed of fluviatile deltaic siliciclastic deposits and lacustrine dolostones the dolostones are enriched in 13 c 13 c 6 6 to 7 6 v pdb 18 o 16 7 to 21 5 v smow which is in general assigned to a global perturbation of the carbon cycle the lacustrine dolostone unit contains desiccated and in situ brecciated carbonate beds subaerial erosion and dissolution surfaces epikarst surficial silicification silcretes and hot water spring travertines the dissolution surfaces marked by formation of a few centimetre thick vuggy non laminated dolomicrite capped by silicified nodular zone resemble modern calcretes which formed by capillary rise of surface waters per ascensum calcrete following saturation of the regolith zone under arid or semi arid conditions abundant red iron stained superficial dolocrete implies the existence of an o 2 rich atmosphere rezolith black pebbles alveolar septal structures microcodium and other fabrics associated with plant roots and input of terrestrial organic matter which are typical for post cretaceous pedogenic calcretes have not been identified alfa fabrics can be applied to describe dolocretes though rhombic calcite crystals are lacking neither 13 c depletion nor 13 c enrichment has been documented in the dolocrete which suggests no contribution of co 2 derived from soil or from any other c org rich sources enriched in 12 c and apparent lack of biological uptake of 12 c during formation of the dolocrete both petrographic and carbon isotope data do not support earlier assertions that palaeoproterozoic subaerial surface was colonised by microbial mats a few samples have high 18 o values up to 30 despite diagenetic and metamorphic resetting of the oxygen isotope system suggesting evaporation during formation of the superficial dolocrete such documented episodes of subaerial exposure formation of hot water travertines superficial dolocrete and silcrete suggest that there must have been frequent decoupling between the kuetsjarvi depositional system and the bordering sea this imposes an additional restriction on the use of the isotopic composition of the ksf dolostones for reconstruction of the 13 c carb global secular curve

microcodium an extensive review and a proposed non rhizogenic biologically induced origin for its formation abstract microcodium has been previously described as a mainly cenozoic calcification pattern ascribed to various organisms a review of the available literature and our data reveal two peaks in microcodium abundance the moscovian early permian and the latest cretaceous paleogene a detailed analysis of late paleozoic and cenozoic examples leads to the following new conclusions typical microcodium forming unilayered corn cob aggregates of elongated grains and thick multilayered palisade replacing structures cannot be linked to smaller grained intracellular root calcifications as became widely accepted after the work of klappa klappa c f 1979 calcified filaments in quaternary calcretes organo mineral interactions in the subaerial vadose environment j sediment petrol 49 955 968 typical microcodium is recognized from the early carboniferous with doubtful devonian reports to quaternary as a biologically induced mineralization formed via dissolution precipitation processes in various aerobic ca rich soil and subsoil terrestrial environments morphology and 13c signatures of microcodium suggest that neither plants algae or roots and root associated mycorrhiza regulate the formation of these fossil structures non recrystallized microcodium grains basically consist of slender 1 5 4 m curved radiating monocrystalline prisms with occasionally preserved hyphae like morphology thin 0 5 3 m hypha like canals can also be observed these supposed hyphae may belong to actinobacteria however thin fungal mycelia cannot be excluded we propose a model of microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and caco3 reprecipitation into the microcodium structures

the late ordovician early silurian glacial period abstract late ordovician to early silurian tillites and evidence of extensive glacial erosion were first discovered in the western sahara in the late 1950 s although the concept of a glacial period at this time had been around since the late 19th century further evidence of glaciation is now available from west africa south africa the arabian peninsula eastern north america west and central southe america and europe several glacial episodes can be distinguished the earliest evidence local mountain glaciation appears to be of caradoc age the major episode may have begun in caradoc time and lasted much of ashgill time when a major ice sheet covered at least north central and west africa which was then centered near the south pole however independent palaeontological and sea level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 ma this conflict has yet to be resolved the latest evidence of glaciation is wenlock in age but is confined to south america which by then was replacing africa over the south polar region however this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice sheet dimensions the lack of land in the north polar region probably explains the absence of tillites from the then northern hemisphere land masses the total area affected by glaciation in early palaeozoic time in north africa alone is at least 6 106 km2 combined with other glaciated areas glaciation may have been on the scale of that of present day antarctica

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

deep source co2 in natural waters and its role in extensive tufa deposition in the huanglong ravines sichuan china abstract the aqueous geochemistry of huanglong ravine sichuan china where tufa is being deposited extensively is discussed using chemical composition data together with the carbon and oxygen stable isotope ratios the water quality of tufa depositing surface streams in this area is basically controlled by the mixing of two source waters fault bounded spring water and snow and or glacier melt water the spring water contains high concentrations of co 2 supplied via faults the resulting high concentration of dissolved co 2 gives rise to much greater dissolution of carbonate rocks than is usual in karst springs and also to the dissolution of silicate rocks upstream spring waters in the ravine are close to equilibrium with respect to calcite concentrations of dissolved inorganic carbon dic and their 13 c values result from the interaction of about 0 02 mol l 1 co 2 aq of 13 c 3 with carbonate rocks of 13 c 3 under closed system conditions about 70 of the co 2 is estimated to be supplied from the upper mantle the data for all the springs fall in the kaolinite stability field in agreement with observed dissolutional aggressiveness towards na and ca feldspar and that the dissolution rates of these minerals are too slow to move the composition of these waters into the feldspar stability field the dic species in the surface streams are in isotopic equilibrium variations in 13 c dic found at different sites are mainly due to differences in the degree of co 2 degassing from the water there is a linear relationship between 18 o values and the altitude at which samples were collected the oxygen isotope composition of surface stream water in this area is controlled by evaporation water chemistry in daytime and nighttime and daily change of ph in the surface water flowing on the tufa deposition indicate that biological activities contribute to the carbonate precipitation although their extent is not great the daily precipitation rate in this area is estimated to be 4300 kg as caco 3 day 1 over a surface area of 0 9 km 2 i e 0 1 cm year 1

earliest laterites and possible evidence for terrestrial vegetation in the early proterozoic paleosols preserve information about the composition of the atmosphere and paleoclimatic conditions here we report the discovery of the first pisolitic laterites of precambrian age the laterites are immediately above the regional unconformity at the base of the early proterozoic gamagara formation in paleokarst depressions on the campbellrand dolomite transvaal supergroup south africa the paleosol profiles are very well preserved and show zoning relict soil textures and chemical composition virtually identical to those of modern laterites the pisolitic laterites provide not only evidence for a highly oxygenated atmosphere and possible hot and humid climatic conditions in early proterozoic time but also indicate the presence of terrestrial life on the kaapvaal craton 2 0 2 2 b y ago

classification and environmental models of cool freshwater tufas abstract tufa deposits are common in the quaternary record facies developments are dependent upon localised carbonate precipitation generally these take the form of phytoherm build ups together with associated detrital deposits derived from disagragation of the former phytoherm cements differ from speleothems and floe calcite accumulations in being biologically mediated or modified this paper recognises the following five types perched spring line cascade fluviatile paludal and lacustrine facies each type is composed of distinctive constructional and detrital sediment associations these unique sediment combinations are the product of an interplay between angle of slope water discharge rate standing water bodies phytal and microbial colonization and ambient temperature the proposed models will be of value in the interpretation of ancient and partially exposed tufas

the role of detrital composition and climate on the diagenetic evolution of continental molasses evidence from the cambro ordovician guaritas sequence southern brazil abstract the cambro ordovician molassic guaritas sequence camaqua basin southern brazil comprises alluvial fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine deltaic deposits and andesitic lava flows the sediments display a complex detrital composition derived from plutonic gneissic acidic volcanic and low grade metamorphic source rocks this detrital assemblage was strongly modified by semi arid continental near surface diagenesis early cementation by hematite smectite quartz and calcite and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones whereas the absence of early cements and or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications the diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition as well as the tectonic provenance classification of the sandstones detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones

carbon isotope record of late cambrian to early ordovician carbonates of the argentine precordillera a more than 2000 m thick cambrian ordovician carbonate platform succession developed on the exotic argentine precordillera terrane during rifting from laurentia and drifting towards gondwana on base of these carbonates a carbon isotope curve could be developed for the cambrian ordovician we measured 13ccarb and 13corg values on bulk rocks selected components and diagenetic cements whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis most subtidal carbonates exhibit primary marine values we can report reliable curves for the middle late cambrian transition and for the latest cambrian to earliest middle ordovician the 13c curve matches well with the published data from global cambrian ordovician boundary sections excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy this indicates the interdependence on sea level which rules the productivity and or preservation of organic carbon and therefore the partition between corg and ccarb burial

life on land in the proterozoic evidence from the torridonian rocks of northwest scotland the stoer group and diabaig formation of the torridonian succession in northwest scotland are late mesoproterozoic to early neoproterozoic ca 1200 1000 ma features preserved on the top surfaces of fine to medium grained sandstone beds in a number of stratigraphically and geographically separated localities are attributable to microbially induced sedimentary structures these include wrinkle structures remnants of apparent microbial crusts and indications of original cohesiveness and pliancy in sand sized sediment the surfaces on which the microbial structures formed were exposed subaerially abundant deep desiccation cracks and locally pedogenic structures in alluvial interfluve and lacustrine margin settings and many of the structures developed in areas well away from the perennially wetted regions adjacent to shorelines and fluvial channels thus these features indicate that earth9s biosphere had adapted to and colonized land surfaces many hundreds of millions of years before the dawn of the phanerozoic

palaeoproterozoic travertines of volcanic affiliation from a 13c rich rift lake environment abstract the ca 2 2 ga kuetsjarvi sedimentary formation ksf of the pechenga greenstone belt the north east fennoscandian shield contains the oldest travertines so far reported in the literature the travertine hosting succession is composed of flat laminated dolomitic stromatolites micritic and allochemical dolostones all formed in coastal plains of a rift related playa lake system in an arid semiarid climate the travertines are categorised as thermal autochtonous spring travertine crust and spring orifice travertine mounds formed from hot water springs issuing at the playa lakeshore the travertines are represented by crusts and small scale mounds both composed of a series of 1 5 mm thick dark light couplets of fibrous dolomite crystals with compromise boundaries and radiating dolomite crystals travertines are composed of dolomite mg ca 0 42 0 47 exhibiting a general lack of replacement textures travertines exhibit several phases of growth separated by dissolution processes and accompanied by precipitation of sio 2 the upper surface of crusts and mounds are often affected by desiccation joints and covered with thicker sio 2 veneer 18 o 18 6 the host dolostones are characterised by 13 c carb of 6 1 8 8 v pdb and 18 o values ranging from 11 to 19 v smow microcored samples of travertine dolomite exhibit 13 c and 18 o values ranging from 6 1 to 7 7 and from 12 0 to 21 5 respectively the 13 c rich nature of the background stratified dolostones is caused by the 2 4 2 06 ga global shift in 13 c 12 c carbonate carbon associated with enhanced accumulation of c org and perturbation of the global carbon cycle the source of co 2 for the travertines is considered to be of volcanogenic origin with an initial 13 c value of ca 6 0 several processes contributed to carbon and oxygen isotope composition of travertine dolomite i an initial co 2 loss at the spring orifice ii water cooling and rapid co 2 loss during water transit iii persistent evaporation from thermal water ponds pools and iv mixing with co 2 of ambient lake water enriched in 13 c silica is thought to have been dissolved by groundwater from underlying sediments and been transported upwards by a mechanism of capillary rise due to dry climatic condition

sequence sequence stratigraphy sea level changes and depositional systems in the cambro ordovician of the north china carbonate platform abstract the cambro ordovician strata in north china were deposited over a very extensive craton extending some 1500 km east west and 1000 km north south the dominantly shallow water carbonate succession reaches up to 2000 m in thickness and two megasequences transgressive regressive cycles can be distinguished lower cambrian through lower ordovician strata and middle through upper ordovician strata separated by a major palaeokarst the first megasequence consists of nine sequences which are generally 50 150 m in thickness the lower cambrian sediments consist of phosphorites and phosphatic sandstones deposited during the flooding of the craton carbonates mudrocks and evaporites were deposited in the early cambrian under an arid climate laying the foundation for the subsequent long period of shallow water carbonate deposition which lasted some 70 m y the middle and lower upper cambrian sequences consist predominantly of mudrocks and storm deposits tempestites in the lower part and oolitic grainstones and tidal flat lime mudstones in the upper part these represent outer mid and mid inner ramp depositional systems respectively of the transgressive and highstand systems tracts these sequences have strong similarities with the grand cycles in the cambro ordovician successions of north america in the upper cambrian there is a distinctive unit of glauconitic purple red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence in the upper cambrian there was a phase of tilting of the north china carbonate platform to the north and storm deposits especially intraclastic conglomerates and hummocky cross stratified grainstones packstones were very common at this time also common in upper middle and lower upper cambrian strata are stromatolitic thrombolitic bioherms several metres in diameter the upper upper cambrian through lower ordovician strata are dominantly fine grained limestones and dolomites deposited in shallow subtidal and inter to supra tidal environments on a low energy epeiric sea platform this part of the succession is the regressive part of the first megasequence so that overall the platform shows an evolution from platform initiation to platform foundation to a ramp depositional system and then an extensive epeiric platform on a small scale the succession is composed of metre scale shallowing upward cycles parasequences arranged into cycle sets of 10 30 m thick the middle upper ordovician megasequence consists mostly of shallow water carbonates with several thick evaporite units minor palaeokarsts and palaeosoils separate the sequences which are composed of metre scale cycles regional uplift affected the north china platform in the ordovician and sedimentation did not resume until the carboniferous there is a good correlation of the two transgressive regressive megasequences described here with the global 2nd order relative sea level curve for the cambro ordovician

speleothems in gypsum caves and their paleoclimatological significance this article highlights the relationship between speleothems growing inside gypsum caves and the particular climate that existed during their development speleothems in gypsum caves normally consist of calcium carbonate calcite or calcium sulphate gypsum and the abundance of such deposits greatly differs from zone to zone observations carried out over the last 20 years in gypsum caves subjected to very different climates italy spain new mexico northern russia cuba argentina highlight wide variation in their cave deposits in arid or semi arid climates the speleothems are mainly composed of gypsum whilst in temperate humid or tropical regions carbonate formations are largely predominant in polar zones no speleothems develop these mineralogical details could be useful paleoclimatic indicators of climate change the interpretation proposed is based on the fact that in gypsum karst the kind of speleothems deposited is determined by competition between the two principal mechanisms that cause precipitation of calcite and gypsum these mechanisms are completely different calcite speleothem evolution is mainly controlled by co2 diffusion while gypsum deposits develop mostly due to evaporation therefore the prevalence of one kind of speleothem over the other and the relationship between the solution precipitation processes of calcite and gypsum may provide evidence of a specific paleoclimate additionally other non common deposits in gypsum caves like moonmilk cave rafts and dolomite speleothems can be used as markers for the prevalence of long dry periods in humid areas seasonal changes in climate or rainfall trends in some gypsum areas moreover the dating of gypsum speleothems could contribute paleoclimatic data relating to dry periods when calcite speleothems are not deposited in contrast the dating of calcite speleothems in gypsum caves could identify former wet periods in arid zones

early forest soils and their role in devonian global change a paleosol in the middle devonian aztec siltstone of victoria land antarctica is the most ancient known soil of well drained forest ecosystems clay enrichment and chemical weathering of subsurface horizons in this and other devonian forested paleosols culminate a long term increase initiated during the silurian from silurian into devonian time red clayey calcareous paleosols show a greater volume of roots and a concomitant decline in the density of animal burrows these trends parallel the decline in atmospheric carbon dioxide determined from isotopic records of pedogenic carbonate in these same paleosols the drawdown of carbon dioxide began well before the devonian appearance of coals large logs and diverse terrestrial plants and animals and it did not correlate with temporal variation in volcanic or metamorphic activity the early paleozoic greenhouse may have been curbed by the evolution of rhizospheres with an increased ratio of primary to secondary production and by more effective silicate weathering during silurian time

hawaiian submarine manganese iron oxide crusts a dating tool black manganese iron oxide crusts form on most exposed rock on the ocean floor such crusts are well developed on the steep lava slopes of the hawaiian ridge and have been sampled during dredging and submersible dives the crusts also occur on fragments detached from bedrock by mass wasting on submerged coral reefs and on poorly lithified sedimentary rocks the thickness of the crusts was measured on samples collected since 1965 on the hawaiian ridge from 140 dive or dredge localities fifty nine 42 of the sites were collected in 2001 by remotely operated vehicles rovs the thinner crusts on many samples apparently result from post depositional breakage landsliding and intermittent burial of outcrops by sediment the maximum crust thickness was selected from each dredge or dive site to best represent crusts on the original rock surface at that site the measurements show an irregular progressive thickening of the crusts toward the northwest i e progressive thickening toward the older volcanic features with increasing distance from the hawaiian hotspot comparison of the maximum crust thickness with radiometric ages of related subaerial features supports previous studies that indicate a crust growth rate of about 2 5 mm m y the thickness information not only allows a comparison of the relative exposure ages of two or more features offshore from different volcanoes but also provides specific age estimates of volcanic and landslide deposits the data indicate that some of the landslide blocks within the south kona landslide are the oldest exposed rock on mauna loa kilauea or loihi volcanoes crusts on the floors of submarine canyons off kohala and east molokai volcanoes indicate that these canyons are no longer serving as channelways for down slope sediment laden currents mahukona volcano was approximately synchronous with hilo ridge both being younger than hana ridge the nuuanu landslide is considerably older than the wailau landslide the waianae landslide southwest of oahu has yielded samples with the greatest manganese iron oxide crusts 9 5 mm thick and therefore apparently represents the oldest submarine material yet found in the study area the submarine volcanic field 100 km southwest of oahu is apparently younger than the waianae landslide

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

carbon dioxide in the paleozoic atmosphere evidence from carbon isotope compositions of pedogenic carbonate stable carbon isotope compositions of pedogenic carbonate occurring in three clay rich vertic paleosols within paleozoic red bed successions in central pennsylvania provide a record of past pedogenic environments and can be used to estimate co sub 2 pressure p sub co sub 2 of the paleozoic atmosphere the delta sup 13 c values of carbonate nodules from paleosols in the deltaic lower bloomsburg formation upper silurian reflect the contribution of carbon from marine groundwater or fossils coupled with low biological activity the delta sup 13 c values of carbonate rhizocretions from stratigraphically high paleosols in the bloomsburg formation and in the alluvial catskill upper devonian and mauch chunk upper mississippian formations suggest an extensive c sub 3 flora and significant contribution of atmospheric co sub 2 paleozoic atmospheric co sub 2 levels inferred from delta sup 13 c of pedogenic carbonate are significantly higher than present levels

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

initial stages of laminar calcrete formation by roots examples from the neogene of central spain thin carbonate laminae formed by root activity are common within miocene detrital deposits of the duero and madrid basins the laminae are about 3 cm thick extending several metres laterally and displace the original detrital sediment which ranges in grain size from fine gravel to sandy clay the thickness morphology microstructure and stable isotope compositions of the laminae indicate that they formed by the activities of roots and associated microorganisms within the soil the laminae are similar to those commonly recognised in thick laminar calcrete profiles three types of laminae are recognised differences in the micromorphology of the laminae are explained as reflecting the different organisms involved and whether calcification occurred when the root was alive or decaying the first type occurs in a profile at paracuellos the jarama where the laminae consist of a mosaic of 20 mm calcite crystals whose arrangement indicates that calcification took place in the medulla of the root and probably occurred while the plant was alive in a second type in the villacadima profile laminae comprise calcified root mats whose formation indicates the interplay of roots and fungi calcification of the cell walls and intracellular spaces took place in the cortices of the roots and not in the medullas as revealed by the presence of central pores in every calcified root a third type of lamina is recognised in profiles at vin egra de moran a and consists of mucilaginous sheets coated by needle fibre calcite crystals the occurrence of associated small root casts indicates that formation of the laminae occurred while the root was decaying and was also influenced by fungal activity these laminae occur within poorly developed soils and their formation was controlled by the relationship between sedimentation erosion and soil formation processes thus the occurrence of these laminae interbedded with detrital sediments reflects environments where sedimentation was relatively low and episodic so after the detrital sediment input surfaces were stable and root mats were able to develop renewed sedimentation accounted for the death of the root mats and the development of new ones on the new surfaces where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminar calcrete profiles form with little or no interbedded detrital sediment

geochemistry and mineralogy of travertine deposits of the sw flank of mt etna italy relationships with past volcanic and degassing activity abstract travertine deposits outcropping in the lower sw flank of mt etna were studied for their mapping as well as for their chemical mineralogical and isotopic compositions these deposits are dated to about 24 to 5 ka in the adrano area located at the western limit of the study area in this area travertines show high mg contents and are composed mostly of dolomite thus apparently ruling out any primary deposition in favour of a diagenetic origin travertines outcropping near paterno in the east part of the study area should be younger than 18 ka those located to the ssw of paterno paterno diga show high sr contents and aragonite as dominant mineralogical phase thus suggesting primary deposition those located to the w of paterno paterno simeto stazione are instead poor both in mg and in sr and show calcite as dominant phase carbon isotope composition of travertines indicates a magmatic origin of co2 that formed them based on the estimated volume of travertines between 10 and 20 gg a of co2 were involved in their formation the time span of travertine formation coincided with the eruptive cycles of ellittico and the first part of mongibello which were probably characterised by a greater amount of co2 transported through groundwater circulation widespread travertine deposition probably ceased after the opening of the valle del bove depression that modified the volcanologic and hydrologic conditions in the summit crater area

periglacial polygonal structures and sand wedges in the late precambrian glacial formations of the taoudeni basin in adrar of mauretania west africa abstract large scale polygonal structures and sand wedges are found over a large area on top of late precambrian continental glacial deposits the exceptionally good preservation of the structures allows comparison with modern periglacial frost wedges filled by wind transported clastic material in a permafrost environment

new evidence for land plants from the lower middle ordovician of saudi arabia macerations of middle ordovician llanvirnian shales from saudi arabia yield an assemblage of spores of probable land plants cryptospores acritarchs and chitinozoa the production of sporopollenin containing sporelike tetrads is considered a fundamental character of the embryophytes because no extant algae produce spores of this type no trilete spores were found at this horizon reinforcing previous assertions that obligate meiotic tetrads predate the earliest trilete spores sporomorph tetrads and dyads in conjunction with cuticlelike fragments were probably derived from terrestrial plants at a bryophyte grade although there are reports of possibly older cryptospores the hanadir assemblage described herein clearly establishes their presence by llanvirnian time

depositional environments and diagenesis in lake parakeelya a cambrian alkaline playa from the officer basin south australia laminated and desiccated siliceous dolostones dolomitic mudstones and dolomitic sandstones in the cambrian parakeelya alkali member of the observatory hill formation accumulated in an alkaline playa four facies are recognized from lake centre to lake edge lake saline mudflat dry mudflat and sandflat facies the facies occur in cycles cycles of tens of metres thickness record the gradual expansion and contraction of the playa superimposed smaller cycles of tens of centimetres thickness record minor oscillations in the position of the strandline in a shallow lake the dominance of saline mudflat dry mudflat and sandflat facies indicates that the lake was rimmed by broad flat areas with negligible relief the high 18o values of primary and penecontemporaneous diagenetic carbonates of 24 to 28 smow indicate strong evaporation of ground and surface waters within the lake system calcite pseudomorphs of the sodium carbonate minerals trona and shortite have 18o values between 19 and 22 5 and contain fluid inclusions with variable salinites and homogenization temperatures up to around 110 c this suggests that the euhedral alkaline evaporites were dissolved by heated waters calcite pseudomorphs then precipitated from a mixed solution formed by the interaction of these incoming fluids with the relatively saline interstitial brines the sodium bicarbonate solutions formed by dissolution of the evaporites would have been dispersed by the basinal brines so that despite the closed drainage further groundwater concentration did not take place

stable isotope variations in the neoproterozoic beck spring dolomite and mesoproterozoic mescal limestone paleokarst implications for life on land in the precambrian proterozoic karst events lowered 13c values by as much as 11 for the 800 ma beck spring dolomite california and as much as 8 5 for the 1 1 ga mescal limestone central arizona relative to the originally deposited carbonate the 13c changes are attributed to input of 13c depleted organic co2 derived from photosynthetic organisms that colonized the ancient land surface the large isotopic shift and its presence at two separate localities suggest that proterozoic karst surfaces were colonized by significant photosynthetic communities with phytomasses possibly approaching those of today

first evidence for cambrian glaciation provided by sections in avalonian new brunswick and ireland additional data for avalon gondwana separation by the earliest palaeozoic abstract the first evidence for cambrian glaciation is provided by two successions on the avalon microcontinent the middle lowest cambrian middle terreneuvian series and fortunian stage stage 2 boundary interval has an incised sequence boundary overlain by a fluvial lowstand facies and higher olive green marine mudstone on hanford brook southern new brunswick this succession in the lower mystery lake member of the chapel island formation may be related to melting of an ice sheet in avalon the evidence for this interpretation is a muddy diamictite with outsized up to 10 cm in diameter proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone that eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with watsonella crosbyi zone fossils 650 km away in avalonian eastern newfoundland a sea level rise within the watsonella crosbyi chron at ca 535 ma may correspond to a unnamed negative 13 c excursion younger than the basal cambrian excursion bace and the zhuce excursion in stage 2 of the upper terreneuvian series cambrian dropstones are now also recognized on the northern gander margin of avalon in continental slope rise sedimentary rocks in southeast ireland although their age early middle cambrian is poorly constrained dropstones in the booley bay formation provide additional evidence for cambrian glaciation on the avalon microcontinent besides providing the first evidence of cambrian glaciation these dropstone deposits emphasize that avalon was not part of or even latitudinally close to the terminal ediacaran cambrian tropical carbonate platform successions of west gondwana

a late archaean palaeoproterozoic 2 6 ga palaeosol on ultramafics in the eastern transvaal south africa abstract this article describes a palaeosol developed on archaean 3 4 ga ultramafics and overlain by sedimentary rocks of the chuniespoort group 2 55 ga the palaeosol is thick mature and complex forming carapaces on palaeohills from base to top it comprises four lithological units 1 carbonated serpentinite 2 black talc rock 3 silcrete dolocrete 4 silcrete breccia in mudstone matrix petrographic and geochemical studies major elements plus v cr ni zn ga y zr nb pge and th indicate that units 1 to 3 represent former saprolites and silicified carbonated saprolites with bedrock textures still preserved unit 4 has been more intensely leached with some admixture of allochthonous sialic material collapse of the original texture and concentration of the immobile elements an irregular ni mineralization in the bottom part of the profiles is described and compared with equivalent modern lateritic deposits a high carbon content suggests the presence of a surficial mat of primitive micro organisms which could have contributed to the stability of the palaeosol on the hilly topography and greatly enhanced the weathering rate the presence of silcrete is indicative of a semi arid climate possibly with alternating wet and dry seasons or of a dry period having succeeded a wet one the absence of fe oxides and the general deficiency of this element in the upper part of the profiles taking into account that the soil was well drained and aerated support the model of an atmospheric o2 level much lower than at present by an order of two magnitudes

pedogenetic and diagenetic fabrics in the upper proterozoic sarnyere formation gourma mali abstract several diagenetic fabrics are described in the uppermost member of the sarnyere formation an upper proterozoic dolomite 700 600 ma micronodular micrite micro sparite showing nodulisation processes orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal porostromata boundstone further evolution in a vadose zone creates a network of cavities which are later filled by cave stromatolites micropopcorn micrite and multilayered cements a rhythmical repetition of facies evidences a polyphased pedogenetic history leading to a 30 m thick highly complex section such numerous and diversified structures are rarely preserved especially in proterozoic rocks but each of them have equivalents in recent examples of caliche soil profiles

calcium carbonate cement caliche in keweenawan sedimentary rocks 1 1 ga upper peninsula of michigan abstract a complex caliche profile and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the lake superior basin during and after continental basaltic volcanism as interflow conglomerates of the portage lake volcanics and as the oronto group in conglomerates the carbonate cement forms discontinuous zones up to 60 cm thick and several meters in length consisting of up to 25 vol carbonate whereas the caliche horizon at the centennial mine shows typical upward grading features characteristic of pedogenic caliche the widespread sparry carbonate cement is less diagnostic it is interpreted also as caliche for the following reasons 1 it predates regional epidote metamorphism 2 a caliche profile has been described from the same strata 3 textures characteristic of caliche such as carbonate rimmed pebbles pendulose calcite on pebbles sand grains floating in a carbonate matrix and exfoliated chlorite 4 possible vadoids 5 the carbonate rich zones are widespread stratigraphically and geographically in basinward inclined alluvial fans and 6 the distribution of these zones and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate several lines of evidence suggest that the prevailing climate may have been arid with a fluctuating perhaps seasonal rainfall pattern required for caliche 1 a paleolatitude about 30 north 2 carbonate filled shrinkage cracks denoting high evaporation 3 intermittent flashy or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud chip conglomerate enormous previously cemented mudstone rip ups and desiccation cracked mud drapes interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi arid intermontane basin flanked by alluvial fans and with a central saline lake

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

tufted microbial cyanobacterial mats from the proterozoic stoer group scotland upward thinning stellate and polygonal carbonate structures are described and compared to modern and ancient stromatolites although tufted microbial mats are common in modern settings they have rarely been described from ancient rocks owing to their poor preservation potential the preservation of these from the stoer group is due to early replacement by calcite before their original tufted relief could be obliterated by compaction associated limestones with some similar features are interpreted as flat to mamillated microbial mats

factors controlling tufa deposition in natural waters at waterfall sites abstract study on calcite precipitation has major implications for both the hydrochemical evolutions of river systems and the global carbon cycle the precipitation of calcite generally requires the water to be 5 10 times supersaturated with respect to calcite which is usually achieved by the removal of co2 formation of waterfall tufa has been often simply described as the result of water turbulence in fast flowing water in this paper the formation mechanisms of waterfall tufa are discussed and a series of laboratory experiments are designed to simulate the hydrological conditions at waterfall sites the influences of the air water interface the water flow velocity and the solid water interface on co2 outgassing and calcite precipitation are compared and evaluated quantitatively the results show that the principal cause of waterfall tufa formation is the enhanced inorganic carbon dioxide outgassing resulted from the sudden hydrological changes occurring at waterfall sites rather than organisms evaporation or the solid water interface the air water interface area and the water flow velocity are greatly increased at waterfall sites as a result of the aeration effect low pressure effect and jet flow effect which greatly accelerate co2 outgassing inorganic co2 outgassing drives the waters to become highly supersaturated with respect to calcite and consequently results in much calcite deposition the solid water interface is less important as the air water interface in affecting calcite precipitation at waterfall sites field measurements showed that conductivity ca2 and hco3 concentrations along tianhe creek and hot creek decrease downstream while ph rises field observations also showed that tufa deposition occurred mainly at waterfall sites

weathering before the advent of land plants evidence from unaltered detrital k feldspars in cambrian ordovician arenites potassium chelation by land plants and the consequent depletion of k ion activity in soil waters make k feldspars unstable in present day soils other factors remaining constant the absence of land plants in pre silurian time would make k feldspars stable dissolution of plagioclase under the action of soil water would be the same in both pre and post silurian time the presence of abundant fresh k feldspars and the absence of plagioclase in the cambrian ordovician arenites of ohio wisconsin minnesota missouri texas wyoming and montana are probably due to the absence of potassium chelating agents in the soils of the time

stable isotope composition of spelean calcites and gaseous co2 from tylicz polish carpathians abstract spelean calcites discovered in fissures and caverns within flysch deposits at tylicz polish carpathians have been examined isotopically together with co 2 rich water from several springs occurring in this area 13 c values of tdc and spelean calcites indicate that caco 3 has precipitated under conditions close to carbon isotope equilibrium with participation of co 2 of deep origin analyses of 18 o suggest the existence of oxygen isotope disequilibrium between water and gaseous co 2 which can be explained by the short time of contact between the co 2 flux and the shallow groundwaters of meteoric origin

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

a middle ordovician drowning unconformity on the northeastern flank of the okcheon ogcheon belt south korea abstract the maggol limestone of ordovician age was deposited in the taebaeksan taebacksan basin which occupies the northeastern flank of the okcheon ogcheon belt of south korea carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the maggol limestone probably culminated in subaerial exposure of platform carbonates in the early middle ordovician earliest darriwilian elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the sauk tippecanoe sequence boundary beneath the middle ordovician succession and its equivalents most in notably north america and north china due to its global extent this paleokarst unconformity has been viewed as a product of second or third order eustatic sea level fall during the early middle ordovician the sauk tippecanoe sequence boundary in south korea however appears to be a discrete marine flooding surface in the upper maggol limestone strata beneath this surface represent by a thinning upward stack of exposure capped tidal flat dominated cycles that are closely associated with multiple occurrences of paleokarst related solution collapse breccias this marine flooding surface is onlapped by a thick succession of thin bedded micritic limestone that is eventually overlain by a middle ordovician condensed section this physical stratigraphic relationship suggest that second and third order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of maggol deposition the tectonic subsidence is also evidenced by the occurrence of coeval off platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias i e the yemi breccia in the basin with continued tectonic subsidence a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform forming a discrete marine flooding surface that may be referred to as a drowning unconformity this tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested thus it is proposed that the taebaeksan basin in the northeastern flank on the okcheon belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early middle ordovician

keweenawan age caliche paleosol in the lower part of the calumet and hecla conglomerate centennial mine calumet michigan the calumet and hecla conglomerate a keweenawan interflow sedimentary unit that is 1 1 b y in age consists at the centennial mine of a felsitic conglomerate upper part and of 1 2 m of siltstone and shale along the base in these fine grained sediments there is a complex submature caliche soil profile represented by an upward increase in calcite and corresponding changes in calcite structures there is also caliche in conglomerate all of this calcite predates the period of regional epidote alteration the caliche is interpreted to have developed in slowly aggrading sediments that were deposited on a rock floored fluvial plain and took place during the initial stages of slope reversal of the lava plain from southerly to northerly this kind of carbonate accumulation in sediments requires a highly seasonal temperate or tropical climate and represents 15 000 yr of weathering the corresponding keweenawan paleolatitude has been reported to be 30 n

carbonate platform growth and cyclicity at a terminal proterozoic passive margin infra krol formation and krol group lesser himalaya india the infra krol formation and overlying krol group constitute a thick 2 km carbonate rich succession of terminal proterozoic age that crops out in a series of doubly plunging synclines in the lesser himalaya of northern india the rocks include 18 carbonate and siliciclastic facies which are grouped into eight facies associations 1 deep subtidal 2 shallow subtidal 3 sand shoal 4 peritidal carbonate complex 5 lagoonal 6 peritidal siliciclastic carbonate 7 incised valley fill and 8 karstic fill the stromatolite rich peritidal complex appears to have occupied a location seaward of a broad lagoon an arrangement reminiscent of many phanerozoic and proterozoic platforms growth of this complex was accretionary to progradational in response to changes in siliciclastic influx from the south eastern side of the lagoon metre scale cycles tend to be laterally discontinuous and are interpreted as mainly autogenic variations in the number of both sets of cycles and component metre scale cycles across the platform may result from differential subsidence of the interpreted passive margin apparently non cyclic intervals with shallow water features may indicate facies migration that was limited compared with the dimensions of facies belts correlation of these facies associations in a sequence stratigraphic framework suggests that the infra krol formation and krol group represent a north to north westfacing platform with a morphology that evolved from a siliciclastic ramp to carbonate ramp to peritidal rimmed shelf and finally to open shelf this interpretation differs significantly from the published scheme of a basin centred on the lesser himalaya with virtually the entire infra krol krol succession representing sedimentation in a persistent tidal flat environment this study provides a detailed neoproterozoic depositional history of northern india from rift basin to passive margin and predicts that genetically related neoproterozoic deposits if they are present in the high himalaya are composed mainly of slope basinal facies characterized by fine grained siliciclastic and detrital carbonate rocks lithologically different from those of the lesser himalaya

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

environmental controls of speleothem mineralogy in a karstic dolomitic terrain soreq cave israel pleistocene speleothems of the soreq cave beth shemesh israel situated within karstic cretaceous dolomitic rocks consist of two major mineralogical types 1 monomineralic low magnesium calcite lmc 0 5 and 7 5 mol mgco 3 straw stalactites stalagmites helictites and pool deposits 2 mineralogically heterogeneous deposits in the form of conical stalactites flowstones and cave corals produced from thin flowing solution films or sprays in which lmc alternates with high magnesium calcite hmc 13 to 33 mol mgco 3 dolomite 33 50 mol mgco 3 and lmc laminations a unique feature of the type 2 is the continuous 0 50 mol range of mgco 3 sem and delta 18 o studies show that all calcites and dolomites grew by direct precipitation from local groundwater solutions lmc formation represents equilibrium precipitation from dripping water solution with mg ca 2 whereas hmc and dolomite precipitation occurred from solution films enriched in mg as the result of prior clos

life on land in the precambrian microfossils have been discovered in cavity fill and replacement silica that occurs between chert breccia clasts in 1200 million year old paleokarst at the top of the mescal limestone central arizona and in 800 million year old paleokarst at the top of the beck spring dolomite southeastern california microbial communities on precambrian g550 million years ago land may have been extensive enough to affect weathering erosion sedimentation and geochemical processes

evolution of calcrete in palaeodrainages of the lake napperby area central australia abstract carbonate lithofacies developed along the trunk valley of palaeodrainage channels as well as those adjacent to gypsite facies in the lake napperby drainage basin in central australia show wide ranges of sedimentary characteristics reflecting significant variation in depositional environments sedimentological and geochemical characterization of the calcrete lithofacies in this drainage area indicate that whereas variations in bedrock morphology are important in overall distribution of the phreatic and vadose calcrete varieties faulting and basin adjustments since the early quaternary may also have played a significant role in redistribution of the hydrologic zones and hence the evolution of these calcretes accordingly phreatic calcretes which initially precipitated from laterally moving groundwaters along the palaeodrainage channels undergo systematic mineralogical changes due to direct evaporative precipitation and diagenetic displacement replacement processes mainly dolomitization and silicification under present hydro chemical conditions within the zone of grounwater fluctuations however lateral and downward shifts in groundwater flow due to excessive volumetric expansion and reduction in effective permeability of calcrete may ultimately result in isolation of the calcrete like those observed from the eastern margins of the lake the evolution of the vadose calcretes is thereafter topographically controlled and relates to mobilization and redistribution of earlier deposited calcretes under present meteorological conditions

the properties genesis and environmental significance of calcitic pendents from the high arctic spitsbergen abstract calcitic pendents occurring beneath coarse clasts were investigated from a well drained profile near ny alesund spitsbergen developed on fluvio glacial deposits with a few carbonate rock fragments micromorphological study was combined with mineralogical geochemical and isotopic investigations to determine the relationships of calcitic pendent characteristics to soil conditions calcitic pendents are observed at all depths beneath clasts and display a succession of five types of laminae with strongly differentiated overall characteristics the number of laminae and their specific attributes are nearly unchanged with depth and with the petrographic nature of the host clast each lamina consists of weakly differentiated micro laminae that are grouped into six morphological facies precipitation of pedogenic carbonates is shown to be currently active throughout the profile induced by supersaturation of solutes with respect to calcite during the initial stage of freezing this process would form beneath clasts the calcitic pendents that are suggested to successively grow at all depths following the gradual progression of the frost front carbonate dissolution appears to mainly affect the fine soil mass from the surface down to the permafrost level controlled by summer biological activity differences of isotopic values of the whole calcitic pendents from those that would precipitate in equilibrium with present day soil conditions are explained by changes in soil conditions during the accretionary process 14 c values suggest that accretion of the pendents integrates phases which occurred during earlier episodes than the holocene the complex layering of calcitic pendents provides a sequential record of evolving soil forming conditions during a time period possibly longer than the last glacial cycle previous conditions are interpreted in relation to present conditions of precipitation the succession of micro laminae is thought to reflect minor climatic fluctuations and marked changes between adjacent laminae are thought to indicate significant climatic changes