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

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

evidence for initial calcite aragonite composition of lower algal chert member ooids and stromatolites paleoproterozoic gunflint formation ontario canada the chert members of the paleoproterozoic gunflint formation ontario canada are commonly regarded as examples of primary silica chemical sediments this interpretation is founded upon the ubiquitous nature of silica high fidelity of preservation of microfossils found within and lack of observed primary carbonate previous arguments for silica replacement of carbonate are based on indirect evidence and are largely dismissed thus gunflint microfossils are regarded as having lived in a silica rich environment which makes them unusual compared to most other pre phanerozoic microfossils that are known from silicified carbonates we present evidence that carbonate was a primary mineral species of the lower algal chert member where most of the microbiota are found most compelling are iron stained low mg calcite ooids containing 1 well preserved growth laminae 2 textures indicating a disrupted tan gential fabric of growth laminae and 3 quartz crosscutting the growth laminae these indicate the initial mineral was a carbonate possibly aragonite less compelling evidence is found in stromatolite columns specularite crystals in low mg calcite display crosscutting relationships with calcite and quartz indicating that calcite was primary specularite sec ondary and quartz tertiary the crosscutting relations show that silicification took place in at least two stages one of the silicification events took place very early in diagenesis thus the gunflint microbiota may not have existed in a radically different environment silica precipitating than those of most other known proterozoic microbiota high fidel ity of preservation of microfossils does not always indicate that the mineral entombing them was the primary mineral precipitated from aqueous solution

petrological and stable isotope studies of carbonate and sulfide minerals from the gunflint formation ontario evidence for the origin of early proterozoic iron formation abstract the 1 9 ga gunflint formation in the thunder bay district of ontario principally comprises iron formation overlain by an iron poor limestone dolostone member the upper limestone member rocks are virtually unmetamorphosed retaining primary textures except locally around diabase sills the gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron formation and to study the changes from iron rich to iron poor sedimentation of the carbonate minerals in iron rich units siderite formed at or near the sediment water interface with ankerite and calcite forming later during diagenesis and during metamorphism around sills heaviest 13c values of siderite and of dolomite in the upper limestone member are consistently near 0 throughout the stratigraphic section indicating that marine bicarbonate was the source of carbon a spread of values between 0 and 7 for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis whereas 13c values more negative than 7 are the result of metamorphism the heaviest 18o values from unmetamorphosed rocks 5 3 pdb for siderite 6 1 for ankerite and 6 7 for calcite are considered to represent the original marine composition lighter values to 17 represent isotopic exchange reactions with pore fluids at higher temperatures and or isotopic exchange with 18o depleted meteoric water low s c ratios and a relatively narrow range of positive 34s values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean locally higher sulfide contents and a wider range of 34s values were the product of the introduction of sulfur after deposition it is proposed that the gunflint formation was deposited in a stratified water column with anoxic bottom water the depositional basin had restricted communication with the open ocean and was affected by distal volcanism hydrothermal activity associated with the volcanism provided a large source of dissolved iron and possibly silica which helped buffer o2 and sulphate in water to low levels low concentrations of sulphate limited the generation of h2s that would otherwise have restricted the solubility of iron during periods of increased hydrothermal activity the anoxic oxic water boundary moved upwards permitting transport of iron to the shallow shelf where it was precipitated as siderite iron hydroxides iron silicates or pyrite depending on physico chemical conditions the transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary

dynamic controls on accretion and lithification of modern gypsum dominated thrombolites los roques venezuela meter sized thrombolites coated by well developed zonally differentiated microbial mats have been found growing in the shallow waters depth 1 m of a restricted hypersaline lagoon on the archipelago los roques in venezuela by contrast within the deeper parts of the studied lagoon sedimentation is characterized by several decimeters of organic rich material containing gypsum granules lacking carbonate cementation the lithification of the thrombolites is thought to have proceeded as follows first extracellular polymeric substances eps comprising the microbial mat concentrate ca2 and other metal cations by adsorption from the hypersaline waters second some of these bound metals then serve as nucleation sites for primary calcium carbonate caco3 precipitation third while carbonate phases are forming in some zones of the mat in others zones they are being re dissolved due to the acidity generated through the metabolism of sulfide oxidizing bacteria fourth as the dissolved sulfide is oxidized into sulfate the pore water become saturated with respect to gypsum caso4 2h2o fifth as primary gypsum precipitates within the structures endolithic sulfate reducing bacteria metabolize the sulfate moiety in the mineral phase while simultaneously oxidizing the eps trapped during accretion sixth as microbial eps degradation proceeds the anaerobic oxidation of specific protein fractions of the eps matrix leads to increased alkalinity the partial dissolution of gypsum supersaturation with respect to calcium carbonate and ultimately pseudomorphic aragonite replacement this differs from secondary calcite cements in being enriched in 12c and depleted in minor and trace metals initially associated with the eps the biogeochemical processes occurring in this thrombolite constructing lagoon represent a novel field site for studying the chemical and isotopic processes characterizing early diagenetic gypsum and the role microbes play in its precipitation dissolution and calcification in this regard insights gained from this modern field site will help to better understand mechanisms by which some precambrian microbialites were lithified

pore water evolution during sediment burial from isotopic and mineral chemistry of calcite dolomite and siderite concretions abstract coal measures often contain concretions segregations of diagenetic minerals originally formed within unconsolidated sediments three different types calcite pyrite dolomite pyrite and siderite occurring spatially quite close together in the central pennine region of england vary widely in carbon isotope composition 10 35 13c 21 49 and in major cation chemistry ca mg fe mn within some siderite concretions very high mn fe ratios were found in central subsamples these were also most enriched in 13c the fe mg ratio decreases systematically from centre to edge early shallow to deeper later precipitation the calcite pyrite and dolomite pyrite concretions developed completely prior to significant burial both have high mn fe ratios but negative 13c values calcite 21 49 dolomite 8 67 to 10 48 all of these patterns can be equated precisely with theories of pore water evolution developed on the basis of geochemical investigations of modem sediments microbial processes sulphate reduction methanogenesis contributed significantly as did thermal decarboxylation to siderite precipitated at considerable burial depth mn iv and fe iii acted differentially as oxidants producing co2 and increasing alkalinity the interplay of fresh and marine depositional waters is seen most obviously in the presence or absence of sulphate reduction this controlled mineral type iron sulphide or carbonate as well as isotopic and mineral chemistry

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

stable isotope geochemistry of cherts and carbonates from the 2 0 ga gunflint iron formation implications for the depositional setting and the effects of diagenesis and metamorphism abstract 18 o values for all but three of 97 samples of chert along a 220 km transect through the 2 0 ga gunflint iron formation of the animikie basin north america range from 21 3 to 24 7 o there is no correlation between 18 o and the type of chert i e deeper water lutitic chert shallower water arenitic chert and peritidal algal chert the fact that the gunflint iron formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the values of the chert have not been significantly affected by metamorphism field and petrographic observations and the small 18 o range of all the chert types suggests that silicification lithification and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment water interface the uniform 18 o values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval 20 c from a common parent water having a relatively uniform 18 o the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water or the result of evaporative processes siderite in the deep water banded facies of the gunflint displays a 18 o range of 6 0 to 2 6 o whereas siderite in the more shoreward organic rich shale facies is significantly enriched in 13 c 13 c 2 5 to 0 5 o 18 c of siderite does not appear to have been altered as a result of neomorphic recrystallization and authigenic microsparitic siderite along stylolites displays the same 13 c range as primary and neomorphosed siderite however neomorphic and authigenic siderite are depleted in 18 o by up to 7 o relative to primary unaltered microspheroidal siderite the lower 13 c of banded facies siderite relative to the shale facies siderite does not seem to be the result of thermal decarboxylation or anaerobic oxidation abiotic or bacterial of organic matter but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes the more basinward banded facies siderite precipitated from a 13 c depleted fe 2 rich hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic volcanic hydrothermal activity onto the continental shelf the siderite of the shale facies precipitated from a more near shore shallow seawater mass that was comparatively enriched in 13 c 13 c 0 2 o ankerite in the gunflint iron formation occurs as a replacement of primary siderite in the banded facies and as a replacement of greenalite and chert in the arenite facies many ankerite samples have 13 c values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 o depleted waters resulted in a wide range of 18 o values 13 8 22 5 o the unique mineralogy of the upper most gunflint iron formation calcite serpentine and magnetite compared to the majority of the formation the very low values and overlying diabase sills suggest that the non ferroan nature of the carbonate in the upper gunflint is the result of contact metamorphism

rare earth elements in foraminifera tests abstract the concentrations of nine rare earth elements ree la ce nd sm eu gd dy er and yb have been determined in mixed species assemblages of foraminifera tests taken from atlantic ocean sediment core tops reductive cleaning techniques have revealed that ree are present in three phases in foraminifera tests collected from sea floor sediments ree included in the foraminiferal calcite matrix termed lattice ree ree associated with an authigenic femn rich phase adsorbed onto the surface of the test following the death of the organism termed coating ree and ree associated with alumino silicate detritus termed detrital ree which commonly infills the test chambers after burial in the sediment the concentrations of ree in the nondetrital lattice plus coating and lattice phases have been measured in this study approximately 90 of the ree measured in the non detrital phase reside in the coating phase the remainder being present in the lattice phase these data have been used to investigate the relationship between the distribution of dissolved ree in the ocean and in the coating and lattice phases in addition to the ree the concentrations of mn fe cu al and po4 have been measured as an aid to characterisation of the various phases

mississippian paleocean chemistry from biotic and abiotic carbonate muleshoe mound lake valley formation new mexico u s a discussion hasiuk and lohmann 2008 present a geochemical study of cement and crinoid remains from the much studied mississippian muleshoe mud mound new mexico u s a they deduce mississippian paleocean chemistry from data on altered cement and crinoids that are assumed to have recrystallized in sea water and retained their original marine chemistry it is surprising in this context that they present data for just one brachiopod as brachiopods are accepted as one of the most reliable marine proxies they do not discuss the interpretation of muleshoe brachiopod data made by wu and chafetz 2002 the latter authors differentiate intermound from mound brachiopods by their delta 13c values higher mound values are thought to be due to seepage of 13c enriched waters produced by methanogenic bacteria the muleshoe mound is interpreted by stanton et al 2000 as being deposited in water 200 m deep in the dysoxic oxygen minimum zone omz consequently can muleshoe carbonate influenced by methane seepage and deposited in the omz be regarded as an appropriate choice of material for an open marine proxy

was the deposition of large precambrian iron formations linked to major marine transgressions geochemical evidence suggests the iron and silica rich precipitates that compose large neoarchean and paleoproterozoic iron formations formed along a chemocline between iron rich deep water and iron poor surface water sedimento logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions we suggest this occurred because the chemocline could only impinge on continental shelves during times of sea level rise or highstand on the other hand the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others to explain this we propose that iron concentrations in the precambrian ocean were greatest at mid water depths in a zone on the order of 100 m thick radiometric age dates from the hamersley basin of western australia suggest major iron formation deposition had a periodicity of ca 20 to 125 m y close to second order cycles in phanerozoic sea level driven by long te

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the 1 9 ga gunflint chert the 1 88 ga gunflint biota is one of the most famous precambrian microfossil lagerstatten and provides a key record of the biosphere at a time of changing oceanic redox structure and chemistry here we report on pyritized replicas of the iconic autotrophic gunflintia huroniospora microfossil assemblage from the schreiber locality canada that help capture a view through multiple trophic levels in a paleoproterozoic ecosystem nanoscale analysis of pyritic gunflintia sheaths and huroniospora cysts reveals differing relic carbon and nitrogen distributions caused by contrasting spectra of decay and pyritization between taxa reflecting in part their primary organic compositions in situ sulfur isotope measurements from individual microfossils 34sv cdt 6 7 to 21 5 show that pyritization was mediated by sulfate reducing microbes within sediment pore waters whose sulfate ion concentrations rapidly became depleted owing to occlusion of pore space by coeval silicification three dimensional nanotomography reveals additional pyritized biomaterial including hollow cellular epibionts and extracellular polymeric substances showing a preference for attachment to gunflintia over huroniospora and interpreted as components of a saprophytic heterotrophic decomposing community this work also extends the record of remarkable biological preservation in pyrite back to the paleoproterozoic and provides criteria to assess the authenticity of even older pyritized microstructures that may represent some of the earliest evidence for life on our planet

crystal chemical controls on rare earth element concentrations in fossil biogenic apatites and implications for paleoenvironmental reconstructions abstract fossil biogenic apatites display rare earth element ree patterns which have been proposed to reflect environmental and biological controls on past seawater composition these patterns can be separated in two groups 1 patterns that are similar to those of open ocean and epicontinental waters and 2 patterns that exhibit a strong enrichment in intermediate ree bell shaped patterns because ree trapping in biogenic apatites occurs mostly post mortem it is essential to understand the crystal chemical inorganic factors that can influence the fractionation of ree between apatite and water a model is proposed by which partition coefficients of ree between apatite and water for a substitution mechanism can be extrapolated from mineral melt partition data to seawater or sedimentary basin conditions these are compared with available experimental partition coefficients for ree adsorption mechanism and used to discuss ree patterns in fossil biogenic apatites calculations of ree patterns for apatites at equilibrium with water show that the bell shaped patterns in fossil apatites can be explained by fractionation with seawater or continental fluids at low temperature under crystal chemical control involving a substitution mechanism and in the context of extensive or late diagenesis in that case the equilibrium composition of the fluid cannot be unequivocally determined from observed ree concentrations in fossil apatites this type of sample is inappropriate to discuss the past environmental or biological controls on ree concentrations in the ocean other sample compositions suggest little to moderate alteration of the original oceanic to freshwater pattern through the adsorption mechanism in the context of weak or early diagenesis and may be used with caution for that purpose

proterozoic ministromatolites with radial fibrous fabric small digitate stromatolites with diameters in the range of 0 2 20 mm ministromatolites are common in early and middle proterozoic carbonate sequences and extend stratigraphically from the archaean to the holocene an occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial fibrous fabric and microscopically crinkled microcrenate lamination is described from the 1 9 gyr old belcher supergroup mcleary formation in southeastern hudson bay canada the structures which can be considered to be a variety of tufa are unusually well preserved because of early diagenetic silicification columnar types are referable to pseudogymnosolen asperia and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern canada where the fabric is normally preserved by a secondary mosaic of dolomite the textural evidence of angulate cross sections and rectilinear divergent patterns indicates that the radial fibrous fabric represents primary or very early diagenetic precipitation and that pseudogymnosolenids with mosaic dolomite originally also had radial fibrous structure the precipitation may have been within or on microbial mats

sedimentary phosphate and associated fossil bacteria in a paleoproterozoic tidal flat in the 1 85 ga michigamme formation michigan usa abstract phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release concentration and mineralization preserved fossil bacteria in phosphate crusts and grains from the ca 1850 million year old bijiki iron formation member of the michigamme formation michigan provide insight into the longevity and nature of this relationship the michigamme formation accumulated near the end of the earth s initial phosphogenic episode ca 2 2 and 1 8 ga to produce one of the first granular phosphorites phosphatic lithofacies consist of fine to medium sand sized francolite peloids concentrated on bedding surfaces in peritidal facies granular beds are up to 2 cm thick and peloids are often partially to completely replaced by dolomite and chert the grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction the peritidal nature of phosphorite in the michigamme formation is in sharp contrast to phanerozoic phosphogenic environments in deeper coastal upwelling settings peritidal settings were well suited for phosphogenesis under the very low oxygen and low dissolved sulfate levels of the paleoproterozoic as cyanobacteria produced oxygen in shallow water and evaporation led to increased sulfate concentrations such concomitant processes helped establish focused redox interfaces in the sediment that chemosynthetic bacterial communities sulfur oxidizers reducers forms that concentrate p and possibly iron oxidizers could exploit phosphate released from organic matter by heterotrophic bacteria and fe redox pumping was further concentrated by these chemotrophs a process that forms late neoproterozoic to phanerozoic phosphorites but on a much larger scale this early example of a granular phosphorite demonstrates that like their phanerozoic counterparts paleoproterozoic phosphorites are the concentrated indirectly biomineralized products of bacterial communities but unlike younger analogs which accumulated across subtidal shelves and shelf margins these ancient deposits formed only in tidal flat settings where phosphogenic redox processes could be established in the sediment from this early beginning the zone of phosphogenesis likely migrated into deeper water settings as oxygen and sulfate levels rose expanding the zone of chemosynthetic bacterial and associated phosphogenesis across the shelf

restricted shallow water sedimentation of early archean stromatolitic and evaporitic strata of the strelley pool chert pilbara block western australia abstract the 3 4 ga old strelley pool chert is a 25 m thick sedimentary unit near the top of the predominantly volcanic warrawoona group in greenstone belts of the eastern pilbara block western australia it is here subdivided into 5 members containing 13 lithofacies the basal member i is composed of quartzose sandstone deposited in a high energy wave or tide dominated shallow water system overlying this are members ii and iii which make up the bulk of the formation and were deposited in a low energy partially restricted hypersaline basin they record a predominantly regressive succesion of deposits including subaqueous laminite stromatolite and evaporite stromatolite carbonaceous laminite black and white banded chert evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions and subaerially deposited windblown sand evaporite and evaporite solution layers members iv and v record the progradation of a volcaniclastic alluvial fringe the strelley pool chert represents an association of sedimentary environments directly comparable to that observed in modern low energy shallow marine carbonate evaporite systems such as along the trucial coast of the persian gulf and abundantly developed in phanerozoic carbonate platform deposits there is no evidence however that uniquely identifies the environment as having been marine deposition may have taken place in either a large hypersaline lake or a restricted marine basin evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes silicified evaporites including coarsely crystalline layers resembling messinian selenite are widespread and similar to younger evaporite deposits they clearly indicate that evaporites were common within shallow water archean sequences the presence of an assemblage of biogenic deposits including organic laminite stromatolites encrusting carbonaceous mats carbonaceous granules and oncolites deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic points to the existence of an ecologically and probably biologically diverse microbial community 3 4 ga ago

the incorporation of rare earth elements in modern coral we report measurements of rare earth elements rees which show that these trace elements are being incorporated in modern coral in proportion to their seawater concentrations four bermuda north rock coral species diploria strigosa diploria labyrinthiformis montastrea annularis and porites astreoides and two tarawa atoll samples of the species hydnophora microconos were analyzed by ti idms following cleaning techniques to isolate the lattice bound rees based on the replicate analyses of the same piece of diploria strigosa excellent reproducibility was achieved the ree ca ratios 0 1 3 nmol mol of the bermuda and tarawa corals are similar to those of cd ca the trace metal with the lowest seawater concentration used in coral studies with the exception of ce the distribution coefficients e g dnd nd coral nd seawater ca seawater ca coral between bermuda coral lattice and sargasso seawater have fairly flat patterns across the ree series the values of d range from 1 to 3 like those reported for other trace elements in corals this suggests but does not prove that rees are incorporated in the aragonite lattice of these corals the shale normalized ree patterns of the tarawa corals also have seawater like distributions however no local seawater data are available to calculate values of d two bermuda species d labyrinthiformis and p astreoides have values of d for ce that are high with respect to the d values of la and nd implying that there is preferential uptake of ce into the lattice this may be related to the fact that ce is the only ree with an active redox chemistry in seawater there is considerable interest in using the chemical and isotopic composition of coral as indicators of climatic variations the combination of ree ca ratios and neodymium isotopic composition of coral has the potential to help understand important natural processes a primary application could be directed toward a tracer for river water discharge in tropical regions and by inference a proxy for rainfall climate variation and weathering history this application would exploit the fractionated composition of rees in river water as a terrigenous signature in coastal corals

taphonomy of very ancient microfossils from the 3400ma strelley pool formation and 1900ma gunflint formation new insights using a focused ion beam abstract focused ion beam fib milling permits the accurate extraction of ultrathin c 100 nm cross sectional lamellae from microfossils found in geological thin sections subsequent tem analysis of these lamellae can provide unique insights into the ultrastructure chemistry and taphonomy of precambrian microfossils at the micrometer to nanometer scale combining serial fib milling with sem imaging extends this capability to three dimensional 3d tomographic reconstruction and visualization of precambrian microfossils revealing information not available in light microscopy here we apply these techniques to two iconic silicified microfossil assemblages from the 3400 ma strelley pool formation of western australia and the 1900 ma gunflint formation of canada all the examined microfossils have carbonaceous walls surrounded by pure silica impregnation of microfossil walls by nano grains of silica is common together with variable degrees of wall displacement and replacement by silica all microfossils are rigidly preserved in 3d and show little or no folding or compression however there are also notable differences in taphonomic preservation our examples of the spheroidal gunflint microfossil huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano silica grains that resemble those found on bacterial surfaces in modern silicifying hot spring environments the nucleation of these silica nano spheres on the microfossil walls has induced an artificial saw tooth like ridged wall texture that may subsequently hinder species level identification the strelley pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica plus extensive recrystallization of spheroidal silica nano grains to angular micro quartz our examples of the sheath like filamentous gunflint microfossil siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains a model is presented to explain these observations criteria for distinguishing highly probable microfossils from non cellular carbonaceous microstructures e g botryoids and grain coatings using fib based imaging are put forward for the first time here using examples drawn from the strelley pool formation and comparisons with younger gunflint material the combined in situ techniques of fib tem and fib sem nano tomography potentially provide a wealth of new nano scale information regarding the biogenicity antiquity and taphonomy of precambrian microfossils however the destructive nature of both techniques makes their application to unique palaeontological specimens problematical

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

carbon isotopic composition of individual precambrian microfossils ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 ma bitter springs formation australia and the approximately 2100 ma gunflint formation canada the delta 13c pdb values from individual microfossils of the bitter springs formation ranged from 21 3 1 7 to 31 9 1 2 and the delta 13c pdb values from microfossils of the gunflint formation ranged from 32 4 0 7 to 45 4 1 2 with the exception of two highly 13c depleted gunflint microfossils the results generally yield values consistent with carbon fixation via either the calvin cycle or the acetyl coa pathway however the isotopic results are not consistent with the degree of fractionation expected from either the 3 hydroxypropionate cycle or the reductive tricarboxylic acid cycle suggesting that the microfossils studied did not use either of these pathways for carbon fixation the morphologies of the microfossils suggest an affinity to the cyanobacteria and our carbon isotopic data are consistent with this assignment

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

euxinic conditions recorded in the ca 1 93 ga bravo lake formation nunavut canada implications for oceanic redox evolution abstract the composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest paleoproterozoic but the emerging view is that atmosphere ocean system did not experience an irreversible transition to a well oxygenated state instead it has been suggested that the oxygen content of the atmosphere ocean system decreased considerably after ca 2 06 billion years ago ga which resulted in a crash in marine sulfate concentrations the end of the deposition of major granular iron formations at ca 1 85 ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux the record of oceanic redox state is not well constrained for the period between ca 2 06 ga the end of the lomagundi positive carbon isotope excursion and ca 1 88 ga when major granular iron formations appeared we address this gap by presenting new iron speciation major and trace element data as well as sulfur organic carbon and molybdenum isotopic data for greenschist facies organic matter rich mudrocks orms of the ca 1 93 ga bravo lake formation piling group baffin island the iron speciation data suggest deposition of the bravo lake formation under a euxinic anoxic and sulfidic water column trace metal enrichments and mo isotope data suggest extensive marine euxinia ca 90 million years before the disappearance of large scale economic granular iron formations the addition of new mo data in this time interval is important as it contributes to filling in the sparse proterozoic record lastly this work provides further support for the idea that there was widespread anoxia shortly after the end of the lomagundi event

rare earth elements and neodymium isotopes in ferromanganese oxide coatings of cenozoic foraminifera from the atlantic ocean abstract an evaluation has been made of the method of establishing the ree contents and patterns and nd isotopic compositions of sea water over cenozoic time from their record in the femn oxide coatings of foraminiferal calcite using 0 60 ma samples from the rio grande rise dsdp site 357 it has been established that the ree contents of the coatings are generally similar to those of recent samples however in the cenozoic samples the surface coatings have been diagenetically modified under suboxic conditions resulting in a distinctly different ree pattern although the original 143 nd 144 nd ratios appear to have been preserved the nd isotopic curve for cenozoic sea water in the s atlantic shows clear temporal trends although these are not so extreme as to show 143 nd 144 nd ratios outside the range observed in modem sea water with the principal exception of the oldest samples there is an approximate inverse relationship between the nd and sr isotopic compositions of the foraminifera it is suggested that the changes reflect both global changes in the relative proportions of nd and sr derived from continental input and from the weathering of volcanic debris together with short term and local variations to which the sr curve is insensitive reflecting the different response times of the two elements to changes in oceanic input functions the nd isotope curve appears to be a potentially useful tracer of ocean palaeochemistry

geochemistry of late archaean stromatolites from zimbabwe evidence for microbial life in restricted epicontinental seas we report comprehensive trace element and sr isotope data for microbial carbonates from the archaean mushandike limestone masvingo greenstone belt zimbabwe the stromatolites have very coherent ree y patterns and share the essential shale normalised characteristics of other archaean marine precipitates positive la and gd anomalies absence of a negative cc anomaly and a strongly superchondritic y ho ratio mixing models constrain the maximum amount of shale contamination to 0 25 1 and calculated detritus free carbonate ree y systematics require precipitation from seawater in terms of light ree over heavy ree depletion however the studied samples are very different from all other known archaean marine precipitates in shale normalised plots the mushandike samples yield a negative slope a very restricted regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss ree y data yields a pattern closely resembling typical shale normalised archaean marine chemical sediments the disappearance of a negative eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation sr isotope ratios are strongly correlated with trace element contents and ratios which explains the modest scatter in sr isotope ratios as representing minor clastic contamination importantly even the least contaminated samples have very radiogenic initial sr 87 sr 86 ratios 0 7184 implying sr input from an ancient high rb sr source such as the early archaean gneisses of south central zimbabwe a local ancient 3 5 3 8 ga source is also indicated by previously published pb isotope datasets for the mushandike stromatolites this is entirely compatible with the occurrence of 3 7 3 8 ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality comparison of the pb isotope ratios of the mushandike stromatolites with 2 7 and 2 6 ga old stromatolites from the neighbouring belingwe greenstone belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean the development of numerous basins on old continental crust with water masses variably restricted from the open ocean suggests a lack of strong vertical topography on this late archaean craton c 2004 elsevier b v all rights reserved

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

phosphates and carbon on mars exobiological implications and sample return considerations much of the surface of mars may preserve chemical information contained in rocks from the noachian era with ages that overlap the correspondingly earliest archean geological history of the earth or from before around 3800 ma ma 1 106 years metabolically sophisticated life which utilized phosphate and carbon and was capable of fractionating carbon isotopes was apparently present already on earth by 3800 ma or within 600 ma after the formation of the planet an early appearance of life on earth opens the strong possibility for a similarly early and rapid emergence of life on planet mars this hypothesis remains within the realm of plausibility so long as it can be established that liquid water and energy sources were available there for inchoate life and that the life that emerged reached a level of complexity which could be recognized by its chemical and perhaps morphological remains hypotheses to be used in the search for an ancient martian biosphere from future sample return missions are testable by examining the record of life in ancient terrestrial sedimentary rocks including those that contain rare and recognizable physical microfossils morphofossils identified on the basis of their shape alone and stable authigenic biominerals which include carbonaceous matter having characteristically fractionated carbon isotope signatures here termed chemofossils prior to sample return these tests can be applied to the mineral associations of the snc meteorites a group of meteorites believed to have originated on mars recent claims of a biological origin for secondary minerals and their features as well as for trace organic compounds in the martian meteorite alh84001 are derived in part from the interpretation of putative nanofossil shapes and the nature of the associated mineral assemblage in small carbonate deposits of an igneous rock such igneous samples would not normally be the best candidate to search for evidence of past life even on earth investigations of these mineral occurrences in the martian meteorites and of the oldest geological records on earth provide a useful framework for 1 using mineral phase relationships 2 analytical data of stable carbon isotopic distributions and 3 the problematic task of morphofossil interpretations in the search for life via future sample return missions from the ancient surface of mars

do archean chemical sediments record ancient seawater rare earth element patterns rare earth elements ree concentrations of archean and proterozoic chemical sediments are commonly used as proxies to study secular trends in the geochemistry of precambrian seawater in addition similarities in the ree signatures of archean chemical sediments and modern seawater have led researchers to argue that some archean rocks originated as biochemical precipitates i e microbial carbonates in shallow marine e g peritidal environments however terrestrial waters including river water and groundwater also commonly exhibit ree fractionation patterns that resemble modern seawater here we present the seawater like ree data for ground

compartmentalisation of beachface sediments along the southwestern coast of australia abstract identification of littoral cells is a prerequisite for the determination of sediment budgets on sandy coasts in this context the aims of the project were first to investigate the applicability of statistical techniques particularly auto correlation and cluster analysis to the definition of littoral cell boundaries second add detail to sedimentological and environmental observations previously reported for the southwestern australian coast and third provide a basis for future examination of sediment transport in a nearshore environment that is generally subject to low wave energy the alongshore component of sediment compartmentalisation has been determined for the sandy beaches of the study area and relationships between environmental parameters and beachface sediment characteristics have been established using data describing textural and compositional attributes the systematic alongshore variation in sediments was determined from the cluster analysis and auto correlation results from the grouping of sediments were then used in conjunction with geomorphic variables to determine boundaries of six sediment compartments littoral cells are better developed on the southern sector of the coast where shoreline salients and headlands are larger and act as boundaries these barriers limit but do not prevent inter compartment sediment transport the findings of this study provide a context for further investigation of littoral cell dynamics and coastal sediment transport the statistical techniques applied to data describing the alongshore distribution of sediments are readily repeatable in other contexts and have general applicability and offer a rigorous procedure for definition of coastal regions

late quaternary peat formation and vegetation dynamics in a lowland tropical swamp nee soon singapore abstract cores of sediment from nee soon a peat forming freshwater swamp located in the perimarine zone of singapore have yielded a record of vegetation history that comprises parts of the last glacial and holocene periods the evidence includes two conventional and four ams radiocarbon dates the oldest of which is 23 050 330 b p uncalibrated radiocarbon years and variations in the abundances of sub fossil pollen and spores and charcoal the results of a study of the relationship between contemporary vegetation and modern pollen accumulation facilitated interpretation of sub fossil tree pollen data peat formation occurred at nee soon during the last glacial when sea levels on the sunda continental shelf were well below those at present the occurrence of montane pollen types notably podocarpaceae in sediment samples dated to the late glacial and early holocene suggests either temperatures substantially lower than those of the present and possibly humid conditions or cold seasonally dry climates and reduced levels of atmospheric co 2 the abundance of charcoal indicates the widespread occurrence of fires throughout the entire period represented by the cores for the mid holocene the evidence indicates an increased saline influence presumably as a result of relatively high sea levels falling sea levels during the late holocene appear to have preceded the onset of the most recent period of peat formation at nee soon the evidence confirms that long last glacial interglacial records of lowland vegetation history are available from terrestrial locations in the humid core of southeast asia and suggests that the complexity of perimarine swamp dynamics is not captured fully by anderson s model which assumes a close relationship between peat formation and sea level

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

evidence for primary aragonite precipitation lower proterozoic 1 9 ga rocknest dolomite wopmay orogen northwest canada abundant precipitation of aragonite is suggested by well preserved fabrics in dolostones of the lower proterozoic 1 9 ga rocknest formation fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans and cement crusts in cryptalgal tufas aragonite was common on the 1 9 ga rocknest shelf as it is on modern tropical shelves a search for similar marine cement fabrics in other precambrian carbonates should help resolve whether aragonite was an important precipitate from precambrian tropical seas implying an ocean carbonate chemistry similar to today or if aragonite precipitation was restricted to rare times in the past calcite and even dolomite being the main carbonates precipitated as recently suggested by some authors

microbially influenced formation of 2 724 million year old stromatolites nanoscale evidence suggests that the tumbiana formation stromatolites in australia were influenced by microbial activity in the stromatolites clusters of organic globules are closely associated with 2 724 million year old aragonite crystals