comparison of covalently bound aliphatic biomarkers released via hydropyrolysis with their solvent extractable counterparts for a suite of kimmeridge clays abstract fixed bed hydropyrolysis at pressures greater than 10 mpa has the unique ability to maximise the yields of covalently bound alkanes without adversely affecting their stereochemistries in order to compare the yields and compositions of the solvent extractable and kerogen bound hopanes and steranes for source rocks of varying maturity six kimmeridge clay samples jurassic type ii kerogen have been subjected to successive dichloromethane extraction and hydropyrolysis the ratios of free to kerogen bound aliphatic biomarkers increase markedly at relatively low vitrinite reflectance ca 0 45 0 50 with large reductions occurring in the concentrations of bound hopanes and steranes however the fact that the total concentrations of observable hopanes and steranes i e free plus kerogen bound do not decrease until the vitrinite reflectance is greater than ca 0 50 provides evidence that neither oil migration nor aromatisation has begun to occur to a significant extent below this maturity level to the authors knowledge this represents the first study to observe a systematic change in epimer ratios with increasing maturity for hopanes and steranes covalently bound in kerogens indeed the kerogen bound hopanes and steranes undergo the same epimerisation reaction pathways as their free counterparts in the bitumen but they are generally less mature in terms of isomerisation at both ring and side chain chiral centres indeed the range of the moretane hopane index for the hydropyrolysis oils is much larger than that for the bitumens ca 5 times indicating a much greater sensitivity to relatively small changes in maturity the covalently bound hopanes and steranes released via hydropyrolysis have enabled the maturity of a contaminated core from the north sea central graben maturity to be assessed and have showed that it is compatible with the kimmeridge formation

the analysis of oil trapped during secondary migration abstract during secondary migration there is an opportunity for oil to be trapped as fluid inclusions fis within framework grains such as quartz and within diagenetic cements that have a crystalline structure oil saturation on migration pathways remains relatively low so typically fewer oil inclusions get trapped compared with samples from an oil column geochemical analysis of the much smaller amounts of inclusion oil present in samples from interpreted oil migration pathways has been attempted for two samples from the champagny 1 and delamere 1 wells in the vulcan sub basin northern offshore australia a combination of petrographic analysis bulk geochemical inclusion analysis and log evaluation confirmed that both samples were from oil migration pathways despite the small number of oil inclusions reliable geochemical data were acquired from both samples that were significantly above the levels detected for the system and outside rinse blanks the fi oil trapped on the interpreted oil migration pathway in champagny 1 was generated from clay rich marine source rock with little terrigenous organic matter input it was generated at peak oil window maturity and correlates best with oils derived from the late jurassic lower vulcan formation in contrast the delamere 1 fi oil contains evidence of greater input of terrigenous organic matter and was generated at early oil window maturity this fi oil also contains a signature of a biodegraded component which could have been generated either from the middle jurassic plover formation or from an older source rock these data indicate that it is feasible to geochemically map migration pathways across prospects or basins and to analyse palaeo oil compositions in oil zones where the few inclusions get trapped this also suggests that the few oil inclusions that sometimes occur in proterozoic or archaean rocks may be analysable in the future which would provide relatively pristine and robust data on the composition and diversity of earth s early biosphere

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

reappraisal of hydrocarbon biomarkers in archean rocks hopanes and steranes found in archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes but the syngeneity of these hydrocarbon biomarkers is controversial to resolve this debate we performed a multilaboratory study of new cores from the pilbara craton australia that were drilled and sampled using unprecedented hydrocarbon clean protocols hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit but when they were detectable they had total hopane 37 9 pg per gram of rock and total sterane 32 9 pg per gram of rock concentrations comparable to those measured in blanks and negative control samples in contrast hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389 5 pg per gram of rock and 1 039 pg per gram of rock respectively polycyclic aromatic hydrocarbons and diamondoids which exceed blank concentrations exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1 000 ng per gram of rock in hydropyrolysates from the ultraclean cores these results demonstrate that previously studied archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons therefore existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by 2 7 billion years ago although suitable proterozoic rocks exist no currently known archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation so future exploration for archean biomarkers should screen for rocks with milder thermal histories

the genesis of dedolomites a discusion based on reactive transport modeling abstract dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of ca rich water this process has been described in the literature as either increasing preserving or decreasing porosity of the initial dolostone according to textures observed in prades ne spain dedolomitization took place due to two simultaneous processes non pseudomorphic replacement of dolomite with calcite at the wall rock of fractures and pseudomorphic replacement in the bulk rock a multicomponent reactive transport model is used to analyze the two dedolomitization processes observed diffusion of ions from the fractures was the driving force for non pseudomorphic replacement in the wall rock the dissolution of dolomite and precipitation of calcite took place in accord with a 1 2 stoichiometry as a consequence of the mineral molar volume the porosity was sealed and replacement limited to a rim on both sides of the fractures according to calculations the development of a rim took less than 500 years advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock due to the absence of empty volume for expansion replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation according to calculations pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters as the infiltrating water was also undersaturated with respect to calcite one stage of calcite dissolution postdated dedolomitization this process was responsible for the widening of open fractures where the dissolution front followed the dedolomitization front into the dolomite matrix intensive calcite dissolution was also developed following the advective flux from the top of the bed downwards drusy calcite is observed to precipitate in the pore spaces which consist of intergrains fractures and dissolution cavities this process is not predicted in detail by reactive transport calculations although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization

riverine evidence for a fractionated reservoir of ca and mg on the continents implications for the oceanic ca cycle analysis of river water rock travertine and soil from the high altitude negligible vegetation setting of the southern tibetan plateau demonstrates that ca and mg isotope ratios are fractionated during weathering dissolved ca in the two rivers studied is derived primarily by limestone dissolution 44 42ca in the rivers averages 0 43 and is statistically distinct from limestone at 0 31 the range in 44 42ca in these small rivers is 0 43 equivalent to the entire range in 44 42ca recorded in marine carbonate over the last 80 ma precipitation of isotopically light travertine with a 44 42ca of 0 21 enriches solute ca in heavy isotopes the mg isotope composition of the rivers is intermediate between limestone and silicate rock averaging 1 5 silicate soil has a 26mg of 0 03 heavier than silicate rock by 0 5 these fractionations in the soil create a companion groundwater reservoir of heavy ca and light mg seasonal variations in ca and mg isotope ratios in the dissolved load are small but define an array which can be modelled as a mixture between a fractionated groundwater reservoir and surface runoff reflecting the isotopic composition of the lithology fractionation of ca during the weathering of the continents is of importance to the global cycle of ca the riverine input of ca to the oceans dominated by carbonate weathering is controlled not only by the composition of the primary continental crust but also by the size and composition of a fractionated reservoir on the continents the impact on the oceanic cycle of ca depends on the relative residence times of dissolved ca in the ocean and the storage time of fractionated ca

the sequence and mechanism of low temperature dolomite formation calcian dolomites in a pennsylvanian echinoderm abstract middle pennsylvanian fossils from the buckhorn asphalt quarry of southern oklahoma are unusual in that early sealing by tar has preserved many of their original textural and mineralogic features and those of the earliest stages of diagenesis echinoderm fragments collected from this locality axe well preserved with original stereom morphology clearly visible optically one to five micrometer anhedral calcian dolomite grains occupy as much as 30 percent of the original stererom of the fragments as displayed by differential etching and mgk x my maps the composition of skeletal fragments is 75 percent ca 98mg 02co3 25 percent ca 54mg 46co3 single crystal x ray di fraction xrd empa tem and stem microanalysis were used to characterize the calcian dolomite phase tem micrographs revealed an approximate 100 a tweed structure identical to that reported by reeder and wenk 1979 our data imply that magnesium in the calcian dolomites was derived largely from the original magnesian calcite of the echinoderm stereom original biogenic high magnesium calcite is altered via local microdissolution reprecipitation to anhedral disordered calcian dolomite grains within low magnesium calcite the product phases low magnesium calcite and calcian dolomite retain the original crystallographic orientation of the stereom these products of early diagenesis are apparent precursors to well defined ordered euhedral microdolomites the relationships between the low magnesium calcite and calcian dolomite and the chemistry and complex ultrastructure of the calcian dolomites support the kind of low temperature mechanism of dolomite formation postulated by gains 1977 goldsm th and graf 1958 and others on the basis of hydrothermal experiments

dolomitization kinetics of hydrothermal bombs and natural settings abstract during high temperature 150 300 c dolomitization experiments the rate of dolomitization increases with temperature reactant surface area reactant solubility ionic strength and mg2 ca2 of the solution one would predict these results from simple kinetic theory the discoveries that so42 slows the rate li increases the rate and the induction period is long for dolomitization could not have been predicted dolomitization is a three step reaction 1 nucleation nucleation of very high mg calcite 35 40 mole mgco3 vhmc or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on caco3 the composition of the vhmc or nonstoichiometric dolomite is a function of mg2 ca2 of the solution other variables such as dissolved fe can affect the composition of the nuclei 2 induction period nucleation occurs during the induction period but the most of this period is post nucleation growth of vhmc nonstoichiometric and stoichiometric dolomite nuclei changes in the solution and substrate surface area affect the duration of the induction period 3 replacement period a caco3 is replaced by vhmc or nonstoichiometric dolomite vhmc nucleates faster than stoichiometric dolomite and therefore begins to replace the reactant first b caco3 and vhmc and or nonstoichiometric dolomite are rapidly replaced by stoichiometric dolomite this phase of the reaction is sensitive to solution variables such as mg2 ca2 and ionic strength the three phase reaction model is consistent with eight characteristics of natural dolomites 1 very ca rich dolomite is common only in modern dolomites 2 there is a direct relationship between mg2 ca2 00 solution and mg ca ratio in the dolomite 3 the stoichiometry of dolomites in some ancient rocks is directly related to the percentage of dolomite in the rock 4 suppression of stoichiometric dolomite nucleation allows the persistence of metastable ca mg co3 phases 5 dolomite limestone contacts are often sharp 6 dolomite selectively replaces fine grained caco3 7 dolomite crystals often have cloudy centers and clear rims 8 dolomite textures are mainly determined by the crystal size of the reactant

a reconstruction of archean biological diversity based on molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup hamersley basin western australia abstract bitumens extracted from 2 7 to 2 5 billion year old ga shales of the fortescue and hamersley groups in the pilbara craton western australia contain traces of molecular fossils based on a combination of molecular characteristics typical of many precambrian bitumens their consistently and unusually high thermal maturities and their widespread distribution throughout the hamersley basin the bitumens can be characterized as probably of archean age accepting this interpretation the biomarkers open a new window on archean biodiversity the presence of hopanes in the archean rocks confirms the antiquity of the domain bacteria and high relative concentrations of 2 methylhopanes indicate that cyanobacteria were important primary producers oxygenic photosynthesis therefore evolved 2 7 ga ago and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere moreover the abundance of cyanobacterial biomarkers in shales interbedded with oxide facies banded iron formations bif indicates that although some archean bif might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria those in the hamersley group formed as a direct consequence of biological oxygen production biomarkers of the 3 methylhopane series suggest that microaerophilic heterotrophic bacteria probably methanotrophs or methylotrophs were active in late archean environments the presence of steranes in a wide range of structures with relative abundances like those from late paleoproterozoic to phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late archean 900 ma before visible fossil evidence indicates that the lineage arose sterol biosynthesis in extant eukaryotes requires molecular oxygen the presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least 1 of the present atmospheric level pal and may have been sufficient to support aerobic respiration

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

the partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism glen roy scotland garnet porphyroblasts in sillimanite bearing pelitic schists contain complex textural and compositional zoning with considerable variation both within and between adjacent samples the sillimanite bearing schists locally occur in regional barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism garnet in these dalradian rocks from the scottish highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution reprecipitation followed by partial dissolution individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co exists with those containing largely unmodified characteristic bell shaped mn profiles this highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite producing reactions the modification of garnet via a dissolution reprecipitation process releases ca into the effective whole rock composition displacing the pressure temperature positions of subsequent isograd reactions this represents the first report of internal metasomatism controlling reaction pathways the behaviour of garnet highlights the importance of kinetic factors especially deformation and fluids in controlling reaction progress and how the resulting variability influences subsequent prograde history the lack of a consistent metamorphic response within and between adjacent schists suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions

millimeter scale concentration gradients of hydrocarbons in archean shales live oil escape or fingerprint of contamination abstract archean shales from the pilbara in western australia contain biomarkers that have been interpreted as evidence for the existence of cyanobacteria and eukaryotes 2 7 billion years ga ago with far reaching implications for the evolution of earth s early biosphere to re evaluate the provenance of the biomarkers this study determined the spatial distribution of hydrocarbons in the original drill core material rock samples were cut into millimeter thick slices and the molecular content of each slice was analyzed in core from the hamersley group 2 5 ga c alkanes had gradually increasing concentrations from the surfaces to the center of the rock while the abundance of steranes hopanes and c 15 alkanes decreased with distance from the outer surfaces in samples from the fortescue group 2 7 ga hydrocarbons were overwhelmingly concentrated on rock surfaces two mechanisms are proposed that may have caused the inhomogeneous distribution diffusion of petroleum products into the rock contamination model and leaching of indigenous hydrocarbons out of host shales driven by pressure release after drilling live oil effect to test these models the hydrocarbon distributions in the archean shales are compared with artificially contaminated rocks as well as younger mudstones where leaching of live oil had been observed the results show that chromatographic phenomena associated with live oil escape and contaminant diffusion have strong effects on molecular ratios and maturity parameters potentially with broad implications for oil source rock correlation studies and paleoenvironmental interpretations for the archean shales the live oil effect is consistent with some of the observed patterns but only the contamination model fully explains the complex chromatographic fingerprints therefore the biomarkers in the pilbara samples have an anthropogenic origin and previous conclusions about the origin of eukaryotes and oxygenic photosynthesis based on these samples are not valid however the study also identified indigenous molecules the spatial distribution of particular aromatic hydrocarbons suggests they are syngenetic although devoid of biological information these aromatics now represent the oldest known clearly indigenous terrestrial liquid hydrocarbons

applications of diffusion data to high temperature earth systems diffusion in general is such a widespread phenomenon in nature that a basic knowledge can serve very effectively to address well known applications not only to thermochronology and petrology but also to environmental geology e g paleoclimate sedimentology and applied geosciences e g assessing contamination of gas stations indeed the list of potential applications of diffusion phenomena is so long that an exhaustive treatment would quickly exceed the scope of this review the reader is therefore referred to reviews covering various aspects of diffusion in the earth sciences including diffusion in melts watson 1994 chakraborty 1995 diffusion processes during diagenesis berner 1980 krom and berner 1980 boudreau 2000 and diffusion phenomena in general e g lasaga 1998 ganguly 2002 watson and baxter 2007 zhang 2008 all of these treatments are based on the same general concepts of diffusion which do not need restating here in great detail applications of diffusion profiles in igneous systems have been thoroughly reviewed a recent reviews in mineralogy and geochemistry volume rimg 69 putirka and tepley 2008 and there is also an excellent volume on low temperature thermochronology rimg 58 reiners and ehlers 2005 describing the impact and application of diffusion theory for age and thermal history determination with these precedents in mind we focus the present review on the application of diffusion in high temperature regimes i e systems at t 500 c both igneous and metamorphic systems are addressed but because of the recent precedents noted above we emphasize recent developments and challenges in metamorphic studies in a broad sense diffusion describes the statistical non directional movement of molecules or heat through a medium of interest as independent theories of diffusion were developed in the early 19th century and compared with the principles of statistical variance it was

characterization of precambrian kerogens by analytical pyrolysis abstract a selection of precambrian kerogens has been characterized by curie point pyrolysis combined with gas chromatography and gas chromatography mass spectrometry the resulting pyrograms can be classified into two groups the first group of pyrograms is derived from kerogens whose pyrolysis products are dominated by alkanes alkenes and alkadienes whereas the second group contains pyrograms from kerogens that produce complex mixtures of branched hydrocarbons dominated by components at every third carbon number c 9 c 12 c 15 c 18 c 21 and up to c 33 it is suggested that these branched hydrocarbons may remotely represent remains of isoprenoid hydrocarbons or other branched hydrocarbons of the original organisms

linking process dimension texture and geochemistry in dolomite geobodies a case study from wadi mistal northern oman understanding the distribution and geometry of reservoir geobodies is crucial for net to gross estimates and to model subsurface flow this article focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of jurassic host rocks from northern oman field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault related dolomite the stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters 1000 ft and probably regionally and are one half meter 1 6 ft thick based on petrography and geochemistry a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite in contrast the fault related dolomite geobodies are trending along a fault that can be followed for at least 100 m 328 ft and vary in width from a few tens of centimeters to as much as 10 m 1 33 ft petrography geochemistry and high homogenization temperature of fluid inclusions all point to the formation of the dolomite along a normal fault under deep burial conditions during the middle to late cretaceous the high 87sr 86sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement the dolomitization styles have an impact on the dimension texture and geochemistry of the different dolomite geobodies and a modified classification scheme compared to the one from jung and aigner 2012 is proposed to incorporate diagenetic geobodies in future reservoir modeling

exploring the impact of diagenesis on isotope geochemical and microstructural alteration features in biogenic aragonite for the quaternary and neogene aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment conversely pre neogene aragonite is often diagenetically altered and calcite archives are studied instead nevertheless the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material here experiments were performed to understand biogenic aragonite alteration processes and products shell subsamples of the bivalve arctica islandica were exposed to hydrothermal alteration thermal boundary conditions were set at 100 degrees c 175 degrees c and 200 degrees c these comparably high temperatures were chosen to shorten experimental durations subsamples were exposed to different o 18 depleted fluids for durations between two and twenty weeks alteration was documented using x ray diffraction cathodoluminescence fluorescence and scanning electron microscopy as well as conventional and clumped isotope analyses experiments performed at 100 degrees c show redistribution and darkening of organic matter but lack evidence for diagenetic alteration except in delta 47 which show the effects of annealing processes at 175 degrees c valves undergo significant aragonite to calcite transformation and neomorphism the delta o 18 signature supports transformation via dissolution and reprecipitation but isotopic exchange is limited by fluid migration through the subsamples individual growth increments in these subsamples exhibit bright orange luminescence at 200 degrees c valves are fully transformed to calcite and exhibit purple blue luminescence with orange bands the delta o 18 and delta 47 signatures reveal exchange with the aqueous fluid whereas delta c 13 remains unaltered in all experiments indicating a carbonate buffered system clumped isotope temperatures in high temperature experiments show compositions in broad agreement with the measured temperature experimentally induced alteration patterns are comparable with individual features present in pleistocene shells this study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits the limitations of a study restricted to a single test organism are acknowledged and call for refined follow up experiments

formation of dolomite at 40 80 c in the latemar carbonate buildup dolomites italy from clumped isotope thermometry the temperature of formation of replacement dolomite and 18 o h 2 o of dolomitizing fluid in the latemar carbonate buildup dolomites italy were estimated independently from carbonate clumped isotope thermometry dolomite formed at 42 72 9 11 c 2 standard deviations sd from fluid with 18 o h 2 o that averages 0 3 3 3 vienna standard mean ocean water 2 sd the estimated temperature and 18 o h 2 o are similar to those of modern diffuse flow fluids at mid ocean ridges the kind of fluid that has been proposed previously as the dolomitizing fluid in the latemar buildup based on the trace element compositions of dolomite calcite in limestone preserves original 18 o but records clumped isotope temperatures 44 76 9 11 c 2 sd that are higher than those at which the limestone formed temperature recorded by calcite but not 18 o was likely reset during dolomitization clumped isotope thermometry has great potential for application to studies of burial and diagenesis by retrieving independent estimates of temperature and 18 o h 2 o with uncertainties as low as 5 c 2 standard errors se and 0 75 2 se respectively from a single stable isotope analysis of a carbonate mineral

the origin distribution and hydrocarbon generation potential of organic rich facies in the nonesuch formation central north american rift system a regional study abstract precambrian unmetamorphosed organic rich clastic rocks whilst uncommon in the geologic record are of interest with respect to their potential for hydrocarbon generation the 1 1 billion year old nonesuch formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central north america previous organic geochemical investigations of the nonesuch formation have been limited to isolated samples from the basal portion of the nonesuch formation at a copper mine white pine in northern michigan in the present study 183 outcrop and core samples of the nonesuch formation from northern wisconsin and michigan upper peninsula were collected for sedimentologic and organic geochemical analyses total organic carbon toc values for the samples ranged from 0 0 to 2 5 and showed a strong correlation between organic richness and depositional environment detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies 1 and 2 which could be further distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms whereas the composition of organic petrography 1 kerogens is primarily aliphatic the composition of organic petrography 2 kerogens is dominated by aromatic and or phenolic constituents the geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source input or preservation superimposed over maturity differences is responsible for both organic petrographic and kerogen pyrolysate compositions

release of bound aromatic hydrocarbons from late archean and mesoproterozoic kerogens via hydropyrolysis hydrogen lean kerogens atomic h c 0 4 isolated from the 2 5 billion year old ga mt mcrae shale hamersley group at tom price western australia were studied via hydropyrolysis a continuous flow technique that degrades organic matter in a stream of high pressure hydrogen assisted by a dispersed mo catalyst the hydropyrolysates yielded predominantly phenanthrene and pyrene and higher polyaromatic hydrocarbons and alkylated homologues were generated in low relative concentrations saturated hydrocarbons were not detected the molecular and carbon isotopic compositions of the hydropyrolysates are very similar to aromatic hydrocarbons obtained by solvent extraction of the host rocks because molecular structures covalently attached to kerogen are unaffected by contamination this indicates that both the bound and extractable aromatic fractions are syngenetic with the host rocks therefore the results of the hydropyrolysis experiments provide compelling evidence for preserved bitumen of archean age the very high proportion of nonalkylated polyaromatic hydrocarbons in the hydropyrolysates is consistent with hydrothermal dehydrogenation of the kerogen and a marked concentration difference of pyrene in rock extracts and hydropyrolysates might be explained by hydrothermal redistribution of the bitumen the kerogen and bitumen composition is therefore consistent with models suggesting a hydrothermal origin for the giant iron ore deposits at mt tom price comparison of the archean samples with hydropyrolysates from immature mesoproterozoic kerogens from the roper group mcarthur basin northern territory and with pyrolysis experiments on proterozoic kerogens in the literature suggests that precambrian kerogens are frequently highly aromatic and lipid poor regardless of their degree of thermal preservation

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

crystal chemistry and thermodynamic and kinetic properties of calcite dolomite apatite and biogenic silica applications to petrologic problems abstract sedimentary minerals are generally metastable phases that given time and changing environmental conditions recrystallize to more stable phases the actual pathway of stabilization is governed by a host of kinetic factors unfortunately much of the theoretical and experimental work on thermodynamic and kinetic behavior of sedimentary minerals either has not reached field practitioners in sedimentary petrology or has been conducted under conditions that are difficult to extrapolate to natural sedimentary environments in this paper we review and present new data on the basic crystal chemistry thermodynamic and kinetic properties of calcite dolomite apatite and biogenic silica and discuss the relevance of these data to the solution of geological and geochemical problems the crystal chemistry and structure of a given magnesian calcite exert a fundamental control on its solubility and solid solution behavior and this control can be seen most clearly through comparison of synthetic and biogenic phases thus variations in crystal chemistry and structure through solubility control during diagenesis yield a range of possible stabilization pathways whose documentation is the domain of much field based study experimental work involving dolomite has focused on delineation of phase relations in dry and aqueous systems at moderate to high temperatures determination of reaction pathways followed during dolomitization of calcium carbonate and measurement of reaction rate uncertainties reside in the relevance of these data to the classic problem of low temperature dolomite formation we suggest that the effort must now focus on designing experimental systems that effectively mimic natural environments and yield reaction rate data as a function of temperature and solution composition such an example is presented a primary goal in experimental work involving carbonate fluorapatite has been an understanding of the mechanism of formation of this mineral we review the state of this knowledge and also present the results of ongoing dissolution rate experiments the importance of this work is that it bears directly on the understanding of the role carbonate fluorapatite plays in the biogeochemical cycle of p in the oceans many factors influence the solubilities of opaline silica and the silica polymorphs and solubility plays an important role in controlling silica diagenesis a model is presented that relates changes in sediment properties including density and acoustic velocity to the stages of silica diagenesis the model is applied to sediments of deep sea drilling project leg 63 the discussions of the sedimentary phases calcite dolomite apatite and biogenic silica in this paper point to several directions for future experimental research on sedimentary mineral solution reactions these include emphasis on 1 experimental studies of synthetic sedimentary mineral solution reactions to form a foundation for an understanding of natural mineral solution reactions 2 experimental investigations of mineral reactions in aqueous solutions under conditions of composition temperature and pressure similar to the natural conditions of mineral formation and 3 studies of the surface as opposed to bulk properties of sedimentary minerals in aqueous solutions and their role in reactions governing precipitation and dissolution of sedimentary phases

characterisation of fluid inclusion oil in a mid proterozoic sandstone and dolerite roper superbasin australia detailed geochemical analysis has been carried out on fluid inclusion oils trapped for over 1 billion years in the roper superbasin northern australia in two different geological settings in healed microfractures in a quartz arenite and within partially altered plagioclase laths in a dolerite sill the oil inclusions and co occurring solid bitumens record multiple oil migration events postdating an intrusive episode ca 1300 ma most likely during neoproterozoic basin inversion solid bitumen represents the oldest migration event and occurs in secondary pore space within the bessie creek sandstone and in veins cross cutting the dolerite sill it is interpreted to represent thermally altered oil that may have at least partly been generated by contact metamorphism near the igneous intrusion inclusion oils in both the dolerite feldspar and in the transgranular fractures in the bessie creek sandstone are pristine nonbiodegraded fluids with a great similarity to other bitumens oils and sediment extracts from the roper superbasin biomarkers indicate that the oils were mainly sourced from prokaryotic cyanobacterial organic matter with a small contribution from eukaryotes based on migration distance organic richness geochemistry and similarity of source rock maturity the velkerri formation which directly overlies the dolerite sill and the bessie creek sandstone is the most likely source rock for the higher molecular weight hydrocarbons preserved in the fluid inclusion oils geochemical data indicate that the oil inclusions in clusters in albitised zones of labradorite crystals in the dolerite matrix represent a different migration event from oil inclusions in transcrystalline fractures in vein calcite in the dolerite sill and in transgranular fractures in quartz in the bessie creek sandstone the bessie creek sandstone fluid inclusion oil was generated in the peak to late oil window 0 9 1 0 vre the higher molecular weight c13 proportion of the dolerite feldspar fluid inclusion oil is of lower maturity 0 8 vre than the same molecular weigh range fluid in the bessie creek sandstone fluid inclusion oil both of these oils were most likely generated in the overlying velkerri formation however the volumetrically dominant low molecular weight hydrocarbons in the dolerite feldspar fluid inclusion oils have a higher maturity 1 3 vre and may have been generated from the deeper ca 1600 ma barney creek formation in the underlying mcarthur group this study shows that oil trapped in fluid inclusions has a greater potential to preserve pristine organic geochemical information than porosity bound oils and dispersed kerogen if oil inclusions occur in sufficient abundance the analysis of these tiny oil encapsulations is technically feasible and can give valuable information on the composition of the earth s early biosphere and precambrian petroleum systems

reactive transport modeling of brine reflux dolomitization anhydrite precipitation and porosity evolution abstract both geothermal convection and brine reflux drive circulation of sea water derived fluids through carbonate platforms during early burial but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood this paper describes high resolution reactive transport model rtm simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin with flow focused in the more permeable shallow carbonates in a baseline simulation involving an elongate 25 km wide grain dominated packstone platform brine reflux resulted in complete dolomitization beneath the 5 km wide brine pool in 335 ky the dolomite body then extends downward at c 22 m 100 ky into an underlying broad area of partial dolomitization this process enhances porosity at shallow depth but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux in contrast geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time 60 dolomite after 1 my with very minor associated anhydrite cementation reflux diagenesis is sensitive to platform geometry with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline fluid flow across thermal gradients enhances reaction rates and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25 and associated anhydrite by 90 the depth and rate of anhydritization is sensitive to the geothermal heat flux whereas platform top temperatures affect dolomitization rate reflux diagenesis is also sensitive to brine density which affects both fluid flow and reaction rates sediment permeability and reactive surface area rsa are key intrinsic controls on diagenesis where the permeability structure permits sufficient fluid flow diagenesis preferentially affects more reactive fine grained sediments however as flow rates decline reactions become focused in the more permeable but less reactive sediments simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favors grainstones once active reflux ceases brines continue to flow in the subsurface but this latent reflux causes only minor dolomitization due to prior mg2 consumption at shallow depth

combined micro fourier transform infrared ftir spectroscopy and micro raman spectroscopy of proterozoic acritarchs a new approach to palaeobiology abstract micro scale analytical techniques permit correlation of chemistry with morphology of individual proterozoic acritarchs organic walled microfossils and thus provide new approaches for elucidating their biological affinities a combination of micro fourier transform infrared ftir spectroscopy and laser micro raman spectroscopy was used to investigate the organic structure and composition of individual acritarchs well preserved neoproterozoic acritarchs from the tanana formation australia ca 590 565 ma and mesoproterozoic acritarchs from the roper group 1 5 1 4 ga australia and ruyang group china 1 4 1 3 ga age poorly resolved but certainly 1000 ma and h stretching bands in the 2900 cm 1 region relative to the c c aromatic ring stretching band at 1600 cm 1 this ftir spectrum is consistent with the ftir spectra obtained from algaenans isolated from extant chlorophyte and eustigmatophyte microalgae ftir spectra of leiosphaeridia sp from the tanana formation contain a less intense aliphatic c h stretching band relative to the c c aromatic ring stretching band by comparison the spectra acquired from the mesoproterozoic acritarchs were dominated by c c aromatic ring stretching bands at 1600 cm 1 relative to moderate weak ch3 terminal groups 1345 cm 1 c h aliphatic stretching 3000 2700 cm 1 and c o 1710 cm 1 although some differences in biopolymer composition occurred between species curve fitting of the aliphatic c hx stretching region provides greater insight into the aliphatic structures of the acritarchs the ch2 ch3 intensity ratio can be used to assess the relative chain length and degree of branching organic material in the tanarium conoideum consists of straight long chain hydrocarbons while the other acritarchs contain hydrocarbons consisting of short chains that are highly branched in this study it was found that raman spectroscopy does not provide additional information about biopolymer composition of proterozoic acritarchs but rather offers complementary data regarding the aromaticity and degree of saturation of the macromolecular structure of acritarch cysts

summary of convective storm initiation and evolution during ihop observational and modeling perspective abstract the data rich international h2o project ihop 2002 experiment is used to study convective storm initiation and subsequent evolution for all days of the experiment initiation episodes were almost evenly divided between those triggered along surface based convergence lines and elevated initiation episodes that showed no associated surface convergence the elevated episodes occurred mostly at night and the surface based episodes occurred during the afternoon and evening surface based initiations were mostly associated with synoptic fronts and gust fronts and less so with drylines and bores elevated initiations were frequently associated with observable convergent or confluent features in the rapid update cycle ruc wind analysis fields between 900 and 600 hpa the ruc10 3 h forecast of the precipitation initiation episodes were correct 44 of the time allowing a tolerance of 250 km in space and for the forecast being early by one period however the accuracy was closely tied to the scale of

forward modeling of the effects of mixed volatile reaction volume diffusion and formation of submicroscopic exsolution lamellae on calcite dolomite thermometry this paper reports the results of several thousand analyses of the mg content of calcite from 31 samples from the ubehebe peak contact aureole death valley california all data reported are from metamorphic calcite formed during mixed volatile mineral reactions in which dolomite remained in the rock the mg content generally increases toward the intrusive contact and bend with increasing temperature but it varies strongly indeed probability distributions for each sample are near gaussian possess a relatively small skewness 1 72 to 3 32 and a variance that is a multiple of the estimated measuring uncertainty these findings complicate direct application of the mg content in calcite for use as an accurate thermometer the purpose of the study presented in the second part of the paper is to explore the significance of these systematic variations of mg composition of calcite to aid the interpretation of contact metamorphic temperatures recorded in carbonates we developed forward models to evaluate the effect of growth zoning volume diffusion and the formation of submicroscopic exsolution lamellae 1 m on the measured mg distribution in individual calcite crystals and compared the modeling results to the field data modeled mg distributions were transformed into histograms by taking into account intersection probabilities and random microprobe analyses modeling results reveal that the original prograde mg zoning in calcite crystal will be reset if the calcite crystal is assumed to grow slowly along a prograde path original low mg compositions can only be preserved if the entire grain forms over a small temperature interval as can be expected for infiltration driven mineral reactions it is shown that all three mechanisms combined give an adequate model for the mg content data we demonstrate that mg distributions in calcite grains of the ubehebe peak contact aureole are the consequence of rapid crystal growth in combination with diffusion and exsolution

synsedimentary diagenesis in a cryogenian reef complex ubiquitous marine dolomite precipitation abstract analysis of the synsedimentary diagenetic phases from a pervasively dolomitised cryogenian 650 ma reef complex south australia reveals a fundamentally different style of marine diagenesis to that of phanerozoic carbonates textural evidence from dolomitised and undolomitised lithologies of the oodnaminta reef complex indicates that depositional components were precipitated initially as calcium carbonate aragonite and calcite dissolution textures and preserved fabrics in ooids indicate an originally aragonitic mineralogy early fibrous marine cements that have well preserved fibrous crystallographic fabrics with a length fast optical character are likely to have precipitated as high magnesium calcite however following this very early phase of diagenesis the dominant marine process occurring in these reefs was dolomite precipitation this involved the dolomitisation of earlier depositional and diagenetic phases as well as the precipitation of fibrous dolomite cements these fibrous marine cements include the newly described radial slow fascicular slow radiaxial slow and rhombic dolomite cements the cements have optical properties chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates dolomite precipitation during marine diagenesis in this reef complex suggests that the oceans of the cryogenian were chemically different to those of the phanerozoic abundant dolomite formation appears to be linked to anoxic magnesium rich seawater during this time marine dolomite precipitation under these conditions may explain the abundance of dolomite in the precambrian sedimentary record

sequential basal faults in devonian dolomite nopah range death valley area california dolomite crystals from the lost burro formation devonian in the nopah range eastern california display basal stacking disorder as evidenced by transmission electron microscopy satellites in electron diffraction patterns indicate that stacking of anions and cations is different from that in ideal dolomite this example conforms to the model of basal defects proposed by goldsmith and graf in 1958 to explain nonstoichiometry in dolomite this dolomite from the nopah range was formed by deep burial replacement of micritic limestone and its peculiar superstructure is tentatively attributed to the late diagenetic conditions during replacement

oil source rock correlation of the lower paleozoic petroleum system in the baltic basin northern europe the correlation of lower paleozoic marine source rocks with reservoired oils by biomarkers is complex because of the uniform early phanerozoic biomass bacteria and algae and the lack of land plant and animal input accordingly the main source rocks for the most prolific oil province in the baltic basin are still a matter of debate the 10 source rocks and 15 oil samples from 5 northern european countries bordering the baltic sea basin were analyzed by gas chromatography gc with flame ionization detector gc mass spectrometry gcms and gcms ms to detect acyclic isoprenoids and aliphatic aromatic and nitrogen sulfur and oxygen biomarkers chemometric tools were applied to screen for meaningful source and age related biomarkers and to highlight genetics extended tricyclic terpane ratios c24 tetracyclic terpane c26 tricyclic terpane ratios and relative c29 sterane concentrations are considered the most promising biomarkers in differentiating llandovery shales from cambrian to ordovician alum shale and for correlation with expelled oil the uranium irradiation related c26 c28 triaromatic steroid concentrations provide possible distinguishing criteria for the source potential of the different alum shale units enhanced oil maturation by volcanic intrusion is highlighted by sterane biomarkers and polycyclic aromatic hydrocarbons the alum shale is here considered the main source rock for oil accumulations in lower paleozoic reservoirs of the baltic basin oil seepage occurring in ordovician limestone was mainly generated by the middle cambrian alum shale and middle cambrian sandstone reservoirs were mainly sourced by upper cambrian and lower ordovician alum shale with higher maturity considerations about the assessment of migration distance are based on carbazole concentrations and c29 sterane isomerization advanced studies to unravel detailed lower paleozoic oil source rock correlations are based on meaningful biomarkers offer approaches to significantly reduce the exploration risk in this area and could be applied to similar early paleozoic petroleum systems in other basins

structural characterization of kerogen in 3 4 ga archaean cherts from the pilbara craton western australia hydrogen lean kerogen atomic h c 0 46 isolated from the 3 4 ga strelley pool chert in the north pole area pilbara craton western australia were studied by vibrational spectroscopy fourier transform infrared ftir spectroscopy and raman spectroscopy nuclear magnetic resonance spectroscopy solid state 13 c nmr spectroscopy catalytic hydropyrolysis followed by gas chromatography mass spectrometry hypy gc ms and isotope ratio mass spectrometry irms the kerogen occurs in sedimentary rocks as clasts and clots deposited together with other detrital materials that are finely disseminated throughout a chert matrix the bulk kerogen 13 c values range from 28 3 to 35 8 solid state 13 c nmr spectroscopy and ftir spectroscopy reveals that the kerogen is highly aromatic fa varying from 0 90 to 0 92 and contains only minor aliphatic carbon or carbon oxygenated c o functionalities the raman carbon first order spectra for the isolated kerogens are typical of spectra obtained from disordered sp 2 carbons with low 2 d ordering biperiodic structure the implications of the raman results show low 2 d ordering throughout the carbonaceous network indicate the incorrect usage of the term graphite in the literature to describe the kerogen or carbonaceous material in the warrawoona cherts hydropyrolysates contain aromatic compounds consisting of 1 ring to 7 ring polycyclic aromatic hydrocarbons which were covalently bound into the kerogen as well as alkanes linear branched and cyclic which were most probably trapped in the microporous network of the kerogen these pahs have mainly c1 and c2 alkylation while c3 substitued aromatics are low in abundance and do not show a high degree of branched alkylation for the first time we have shown a correlation between elemental analysis h c atomic ratios raman spectroscopic parameters id1 ig id1 id1 ig and la and the degree of alkylation of bound polyaromatic molecular constituents generated from hypy for archaean kerogens similarities in molecular

reaction mechanism for the replacement of calcite by dolomite and siderite implications for geochemistry microstructure and porosity evolution during hydrothermal mineralisation carbonate reactions are common in mineral deposits due to co2 rich mineralising fluids this study presents the first in depth integrated analysis of microstructure and microchemistry of fluid mediated carbonate reaction textures at hydrothermal conditions in doing so we describe the mechanisms by which carbonate phases replace one another and the implications for the evolution of geochemistry rock microstructures and porosity the sample from the 1 95 moz junction gold deposit western australia contains calcite derived from carbonation of a metamorphic amphibole plagioclase assemblage that has further altered to siderite and dolomite the calcite is porous and contains iron rich calcite blebs interpreted to have resulted from fluid mediated replacement of compositionally heterogeneous amphiboles the siderite is polycrystalline but nucleates topotactically on the calcite as a result the boundaries between adjacent grains are low angle boundaries 10 which are geometrically similar to those formed by crystal plastic deformation and recovery growth zoning within individual siderite grains shows that the low angle boundaries are growth features and not due to deformation low angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth the cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant fe entered the crystal along the twin boundaries dolomite grains many of which appear to in fill space generated by the siderite replacement also show alignment of cores along the calcite twin planes suggesting that they did not grow into space but replaced the calcite where dolomite is seen directly replacing calcite it nucleates on the fe rich calcite due to the increased compatibility of the fe bearing calcite lattice relative to the pure calcite both reactions are interpreted as fluid mediated replacement reactions which use the crystallography and elemental chemistry of the calcite experiments of fluid mediated replacement reactions show that they proceed much faster than diffusion based reactions this is important when considering the rates of reactions relative to fluid flow in mineralising systems

bacterially induced dolomite precipitation in anoxic culture experiments to study the process of microbial mediated dolomite formation growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in lagoa vermelha a hypersaline lagoon in brazil where dolomite precipitation occurs specifically we report the isolation of a particular strain of sulfate reducing bacteria lvform6 from lagoa vermelha sediment which apparently promotes the formation of nonstoichiometric dolomite sulfate reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 c the precipitates have morphologies similar to those observed in lagoa vermelha sediment our results demonstrate that sulfate reducing bacteria can influence dolomite precipitation under controlled lowtemperature anoxic conditions and imply that anaerobic microorganisms can play an important role in carbonate sedimentation they may have been particularly significant in earth s earliest history when a more reducing atmosphere existed

dolomitization anhydrite cementation and porosity evolution in a reflux system insights from reactive transport models significant volumes of world hydrocarbon reserves occur in dolostones and the majority of these reservoirs are interpreted to be of reflux origin we used a two dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization dolomite cementation anhydrite cementation and porosity evolution in a reflux system we tested the sensitivity of reflux dolomitization to brine concentration mesohaline to near halite saturated brines near surface temperature flow rate porosity permeability feedback relationships reactive surface area and the effect of initial porosity permeability heterogeneity simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization and that mesohaline reflux dolomitization is viable reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers replacement dolomitization increased porosity up to 8 but postreplacement reflux resulted in the precipitation of minor dolomite cements overdolomitization anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25 simulated rates of replacement dolomitization by reflux are fast up to three orders of magnitude faster than seawater dolomitization the rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry temperature and reactive surface area were important controls on the rate of dolomitization whereas the feedback relationship between porosity and permeability was a relatively moderate control our reactive transport models predict the general spatial and temporal trends in dolomite porosity and anhydrite observed in some major dolomite reservoirs

a comparison of nanometer scale growth and dissolution features on natural and synthetic dolomite crystals implications for the origin of dolomite abstract this paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy synthetic dolomite was formed by replacement of calcite at 218 c in mg ca cl solutions the first products to form during dolomite synthesis experiments as detected with x ray diffraction xrd are invariably poorly ordered nonideal dolomite as the reaction proceeds the degree of cation order increases yet the products remain relatively disordered upon examination the surfaces of nonideal dolomite are covered with mounds 10 200 nm wide and 1 20 nm high following depletion of the calcite reactant dolomite becomes relatively well ordered and stoichiometric ideal the surfaces of ideal dolomite are covered with broad flat layers separated by steps that measure tenths to tens of nanometers high comparison of natural and synthetic dolomite surfaces after etching in 0 5 h2so4 for 10 20 seconds indicates that high temperature synthetic and low temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite in contrast synthetic and low temperature ideal dolomite surfaces when etched are characterized by flat layers with deep euhedral etch pits these results suggest that despite the wide range in formation conditions natural and synthetic dolomites form by the same growth mechanisms furthermore the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth

organic geochemical characteristics of highly mature late neoproterozoic black shales from south china reappraisal of syngeneity and indigeneity of hydrocarbon biomarkers abstract during the late neoproterozoic to early cambrian period the transition from mainly microbial ecosystems to eukaryotic marine primary productivity was one of the most profound ecological revolutions in earth s history abundant biomarkers have been reported from high maturity late neoproterozoic sedimentary rocks in south china including the cryogenian datangpo and ediacaran doushantuo formations these biomarkers were suggested to be organic molecular evidence of the survival of photosynthetic eukaryotes in palaeo oceanic environments during the snowball earth era to advance the understanding of late neoproterozoic ecosystems and to re evaluate the provenance and validity of the biomarkers fresh drill core was collected from black shales of the datangpo and doushantuo formations in south china in order to ensure the indigeneity of the solvent extracted hydrocarbons their composition was determined by conducting experiments under strict laboratory procedures including using slice experiments on a precision saw that removed external surfaces the distribution of the recovered aliphatic and aromatic hydrocarbons indicates very a high thermal maturity for the analysed black shales which is consistent with the equivalent vitrinite reflectance 2 5 and the lack of specific biomarkers both prokaryotic bacterial and eukaryotic algal organisms are likely producers during the neoproterozoic however no long chain steranes c26 steranes were detected in the interior of any of the samples and only trace amounts of hopanes and pregnanes in just one sample from the doushantuo formation xs 199 dst on the contrary solvent extraction of millimetre thick slices cut from the external surfaces of the black shales reveals strikingly high level of contamination from organic residues on the sample surfaces these contaminants include n alkanes c10 c33 isoprenoids monomethylalkanes aromatic hydrocarbons naphthalene phenanthrene and their alkyl isomers and abundant biomarkers such as tricyclic terpanes hopanes steranes and diasteranes the concentrations of these biomarkers in the exterior sub samples exceed blank concentrations by more than three orders of magnitude due to surficial contamination these results indicate that previously studied late neoproterozoic samples from south china likely contain mixtures of abundant biomarker contaminants and some indigenous over mature hydrocarbons therefore special care need to be taken with regard the existing biomarker evidence from south china that has been used to illustrate the contribution of eukaryotic photosynthesis to marine primary productivity during the snowball earth period no late neoproterozoic rocks from south china within the appropriate thermal maturity window for survival of indigenous biomarkers are currently known but these may be found during future exploration

a search for molecular fossils in the kerogen of precambrian sedimentary rocks abstract hydrous pyrolysis of the kerogen in precambrian sedimentary rocks of a range of ages and thermal histories was used in an attempt to produce normal iso and anteiso isoprenoid sterane and triterpane hydrocarbons whose structures can be related to known biochemical precursors gas chromatography mass spectrometry was used in the selected ion monitoring mode to achieve a sensitivity of one part hydrocarbon in 10 9 of the rock small amounts of some of these hydrocarbon classes were found in certain lower cambrian and upper precambrian sediments with normal alkanes predominating the yields diminished with increasing age even for well preserved rocks where the minerals showed little effect of metamorphism samples with ages greater than 1 6 10 9 years yielded no detectable hydrocarbons spontaneous dehydrogenation of kerogen results in cross linking cyclization and aromatization of organic structures and extensive transformation of molecular fossils little information on the evolution of precambrian life is contained in the organic matter of the rocks studied in this work further advances along this line will depend critically on the discovery of sediments in which the kerogen is better preserved

mollusc and brachiopod skeletal hard parts intricate archives of their marine environment the biogenic carbonate hard parts of fossil bivalves cephalopods and brachiopods are among the most widely exploited marine archives of phanerozoic environmental and climate dynamics research the advent of novel analytical tools has led many workers to explore non traditional geochemical and petrographic proxies and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms these considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum this paper evaluates differences and similarities of seawater versus that of body fluids cephalopods bivalves and brachiopods exert a strong biological control on biomineralization that to some degree may buffer their shell geochemistry against secular changes in seawater chemistry disordered amorphous calcium carbonate precursor phases later transformed to crystalline biominerals may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases a reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo environment the impact of what is commonly referred to as biological factors on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon oxygen strontium and traditionally used element to calcium ratios in particular the often used 13ccarb or the mg ca and sr ca elemental proxies are fraught with problems an interesting new research field represents the analysis calibration and application of non traditional proxies to mollusc and brachiopod hard parts examples include the carbonate clumped isotope 47 approach and the analysis of the isotopes of ca mg n li s or element to ca ratios such as li ca or b ca and rare earth elements based on considerations discussed here a series of do s and don ts in mollusc and brachiopod archive research are proposed and suggestions for future work are presented in essence the suggestions proposed here include experimental work also field experiments making use of recent archive organisms or where possible a reasonable recent analogue in the case of extinct groups moreover the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra structures must guide sampling strategies for geochemical analyses where feasible a detailed understanding of the diagenetic pathways and the application of multi proxy and multi archive approaches should form the foundation of fossil carbonate archive research the uncritical compilation of large data sets from various carbonate shelled organisms collected at different locations is not encouraged

release of kerogen bound hopanoids by hydropyrolysis hydropyrolysis was used to determine the concentrations of kerogen bound hopanoid hydro carbons the samples characterized were a recent sediment priest pot english lake district and three source rock kerogens green river shale draupne shale and jet rock shale the hydropyroly zate of a hopanoid producing bacterium methylobacterium organophilum showed that this procedure does not overly obscure the original isomer compositions of the samples characterized although signifi cant side chain cleavage does occur which is inevitable with any pyrolysis method notable diaerences are observed between the composition of pyrolyzates and bitumen extracts for each of the source rock kerogens the overall level of hopane isomerization within the pyrolyzate is significantly lower than in the corresponding extracts this is particularly the case for the draupne sample the pyrolyzate of which contains 17b 21b hopanes though these are absent in the bitumen diaerences in isomeric com position of the pyrolyzates are in line with the isomer contents of the bitumen extracts the abundance of kerogen derived hopanes is highly variable both in terms of absolute concentration and also relative to the concentration of hopanes in the bitumen this is in part due to maturity diaerences but deposi tional environment and diagenetic conditions may also exert a control on kerogen bound hopane con centration these results show that hydropyrolysis provides valuable data on the concentration of kerogen bound hopanes whilst largely preserving their original isomeric composition 1998 elsevier science ltd all rights reserved

sulfur iron and calcium cycling associated with natural electric currents running through marine sediment natural electric currents running through marine sediments have recently been found to couple oxygen reduction at the surface to sulfide oxidation in deeper anoxic layers here we show that such spatial separation of oxidation and reduction processes causes non conventional sulfur iron and calcium mobilization and reallocation reduced marine sediment was incubated with overlying oxic water and the vertical distribution of solutes and solids was analyzed after 45 150 days as much as 44 of sediment oxygen consumption was driven by electric currents and electrogenic oxidation of sulfide to sulfate with concurrent proton generation resulted in significant dissolution of iron sulfides and calcium carbonates in the anoxic layers of the sediment most of the mobilized iron diffused to the oxic zone where it formed oxidized iron minerals calcium precipitated in the oxic zone as magnesium calcite the electric coupling of biogeochemical processes in distant regions thus generates unique chemical conditions in marine sediments whereby key elements are mobilized and relocated probably along with trace elements and nutrients we suggest that such electrically coupled biogeochemistry flourishes in marine sediments after transient oxygen depletion leaving distinct signatures of such events in the geological record

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

composition and syngeneity of molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup pilbara craton western australia shales of very low metamorphic grade from the 2 78 to 2 45 billion year old ga mount bruce supergroup pilbara craton western australia were analyzed for solvent extractable hydrocarbons samples were collected from ten drill cores and two mines in a sampling area centered in the hamersley basin near wittenoom and ranging 200 km to the southeast 100 km to the southwest and 70 km to the northwest almost all analyzed kerogenous sedimentary rocks yielded solvent extractable organic matter concentrations of total saturated hydrocarbons were commonly in the range of 1 to 20 ppm g g rock but reached maximum values of 1000 ppm the abundance of aromatic hydrocarbons was 1 to 30 ppm analysis of the extracts by gas chromatography mass spectrometry gc ms and gc ms metastable reaction monitoring mrm revealed the presence of n alkanes mid and end branched monomethylalkanes cyclohexylalkanes acyclic isopre noids diamondoids tri to pentacyclic terpanes steranes aromatic steroids and polyaromatic hydrocarbons neither plant biomarkers nor hydrocarbon distributions indicative of phanerozoic contamination were de tected the host kerogens of the hydrocarbons were depleted in 13 c by 2 to 21 relative to n alkanes a pattern typical of although more extreme than other precambrian samples acyclic isoprenoids showed carbon isotopic depletion relative to n alkanes and concentrations of 2 methylhopanes were relatively high features rarely observed in the phanerozoic but characteristic of many other precambrian bitumens molecular parameters including sterane and hopane ratios at their apparent thermal maxima condensate like alkane profiles high mono and triaromatic steroid maturity parameters high methyladamantane and methyldiaman tane indices and high methylphenanthrene maturity ratios indicate thermal maturities in the wet gas gener ation zone additionally extracts from shales associated with iron ore deposits at tom price and newman have unusual polyaromatic hydrocarbon patterns indicative of pyrolytic dealkylation the saturated hydrocarbons and biomarkers in bitumens from the fortescue and hamersley groups are characterized as probably syngenetic with their archean host rock based on their typical precambrian molecular and isotopic composition extreme maturities that appear consistent with the thermal history of the host sediments the absence of biomarkers diagnostic of phanerozoic age the absence of younger petroleum source rocks in the basin and the wide geographic distribution of the samples aromatic hydrocarbons detected in shales associated with iron ore deposits at mt tom price and mt whaleback are characterized as clearly archean based on their hypermature composition and covalent bonding to kerogen copyright 2003 elsevier ltd

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

assessing biomarker syngeneity using branched alkanes with quaternary carbon baqcs and other plastic contaminants biomarker molecules are valuable for the elucidation of ancient microbial ecosystems and the characterization of petroleum source rocks for such studies acquisition of reliable hydrocarbon data and proof of their syngeneity are essential however contamination of geological samples with anthropogenic petroleum products during drilling storage and sampling can be particularly problematic because these hydrocarbons may over print an original indigenous biomarker profile to evaluate the extent of contamination of drill core and outcrop material we studied the distribution of hydrocarbons in 26 rocks from different locations in the world all rocks had petroleum products on their exterior surfaces twenty two samples also contained surficial hydrocarbons derived from polyethylene plastic including branched alkanes with quaternary carbon centers baqcs and alkylcyclopentanes with pronounced even over odd carbon number preference using three examples from the paleoproterozoic tawallah and mcarthur groups in northern australia we show how indigenous biomarkers can be recognized by comparing hydrocarbon distributions between exterior rock surfaces and the rock interior and how infiltration of allochthonous hydrocarbons can be assessed through the spatial distribution of characteristic polyethylene derived hydrocarbons in the rock the methods outlined in this paper give confidence in the interpretation of biomarkers in particularly sensitive applications such as the first occurrences of certain organisms in the geological record and the provenance of organic matter in meteorites

hydrothermal replacement of biogenic and abiogenic aragonite by mg carbonates relation between textural control on effective element fluxes and resulting carbonate phase abstract dolomitization i e the secondary replacement of calcite or aragonite caco3 by dolomite camg co3 2 is one of the most volumetrically important carbonate diagenetic processes it occurs under near surface and shallow burial conditions and can significantly modify rock properties through changes in porosity and permeability dolomitization fronts are directly coupled to fluid pathways which may be related to the initial porosity permeability of the precursor limestone an existing fault network or secondary porosity permeability created through the replacement reaction in this study the textural control on the replacement of biogenic and abiogenic aragonite by mg carbonates that are typical precursor phases in the dolomitization process was experimentally studied under hydrothermal conditions aragonite samples with different textural and microstructural properties exhibiting a compact inorganic aragonite single crystal an intermediate bivalve shell of arctica islandica and open porous structure skeleton of coral porites sp were reacted with a solution of 0 9 m mgcl2 and 0 015 m srcl2 at 200 c the replacement of aragonite by a ca bearing magnesite and a mg ca carbonate of non stoichiometric dolomitic composition takes place via a dissolution precipitation process and leads to the formation of a porous reaction front that progressively replaces the aragonite precursor the reaction leads to the development of porosity within the reaction front and distinctive microstructures such as gaps and cavities at the reaction interface the newly formed reaction rim consists of chemically distinct phases separated by sharp boundaries it was found that the number of phases and their chemical variation decreases with increasing initial porosity and reactive surface area this observation is explained by variations in effective element fluxes that result in differential chemical gradients in the fluid within the pore space of the reaction rim observed reaction rates are highest for the replacement of the initially highly porous coral and lowest for the compact structure of a single aragonite crystal therefore the reaction progress equally depends on effective element fluxes between the fluid at the reaction interface and the bulk solution surrounding the test material as well as the reactive surface area this study demonstrates that the textural and microstructural properties of the parent material have a significant influence on the chemical composition of the product phase moreover our data highlight the importance of effective fluid mediated element exchange between the fluid at the reaction interface and the bulk solution controlled by the local microstructure

transport controlled hydrothermal replacement of calcite by mg carbonates dolomitization is one of the most important diagenetic processes but the reaction rate and time scale of dolomitization remain a topic of controversy we conducted experiments in which the reaction of single calcite crystals with a mg rich fluid at 200 c leads to the formation of a zoned reaction rim consisting of magnesite and for intermediate times ca rich dolomite from detailed documentation of the evolution of the microtexture and chemical composition of the reaction rim we infer a kinetic model for calcite replacement by mg carbonates the chemical gradient for the structure forming elements mg and ca in the reaction rim and the evolution of the rim thickness strongly indicate that the overall reaction rate is controlled by diffusive transport through the porous reaction rim the composition of the product phases is kinetically controlled and records the local composition of the interfacial fluid without requiring oversaturation of the reservoir fluid reactive transport models on dolomitization processes assume that the rate of dolomitization depends on the rate of dolomite precipitation which is contradictory to our experimental evidence modeling carbonate replacement in natural systems requires detailed knowledge on the evolution of the microstructure controlling the physicochemical transport properties of elements in the pore fluid

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

direct physical evidence of dolomite recrystallization the possibility of recrystallization is a long standing barrier to deciphering the genetic origin of dolomites there is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event results from 65 new high temperature dolomite synthesis experiments 1 m 1 0 mg ca ratio solutions at 218 c demonstrate dolomite recrystallization affecting stoichiometry cation ordering and nanometre scale surface texture the data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization which occur by various dissolution precipitation reactions during the first phase induction period no dolomite forms despite favourable conditions the second phase replacement period occurs when ca rich dolomite products with a low degree of cation ordering rapidly replace calcite reactants during the replacement period dolomite stoichiometry and the degree of cation ordering remain constant and all dolomite crystal surfaces are covered by nanometre scale growth mounds the third phase primary recrystallization period which occurs in the experiments between 97 and 100 dolomite is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering the end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat laterally extensive layers the fourth phase of the reaction secondary recrystallization period occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time inferences of recrystallization in natural dolomite based on cation order or stoichiometry of dolomite usually depend on assumptions about the precursor dolomite subjected to recrystallization if it is assumed that the experimental evidence presented here is applicable to natural low temperature dolomites then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization

extreme magnesium isotope fractionation at outcrop scale records the mechanism and rate at which reaction fronts advance isotopic fractionation of cationic species during diffusive transport provides a novel means of constraining the style and timing of metamorphic transformations here we document a major 1 decrease in the mg isotopic composition of the reaction front of an exhumed contact between rocks of subducted crust and serpentinite in the syros melange zone this isotopic perturbation extends over a notable length scale 1 m implicating diffusion of mg through an intergranular fluid network over a period of 100 kyr these novel observations confirm models of diffusion controlled growth of reaction zones formed between rocks of contrasting compositions such as found at the slab mantle interface in subduction zones the results also demonstrate that diffusive processes can result in exotic stable isotope compositions of major elements with implications for mantle xenoliths and complex intrusions

variation in the diagenetic response of aragonite archives to hydrothermal alteration abstract diagenesis of carbonate minerals is ubiquitous throughout the geologic record alteration is initiated immediately after deposition or takes place in the endo and exoskeletons as early as during the lifetime of a given carbonate secreting biota and can continue throughout the burial history of carbonate sediments and rocks variations in the diagenetic response of carbonate archives pose challenges for the reconstruction of past environmental conditions based on proxy data this paper comparatively assesses alteration features of different aragonitic materials by experimentally induced diagenesis a multitude of factors lead to different reactivity and responses of a given aragonite archive and provide insight to the interpretation of diagenetically altered material in the rock record chosen materials used in this study include relatively organic rich samples such as coral skeletons and bivalve shells and organic lean abiotic carbonates such as speleothems and aragonite single crystals obtained datasets include distributions of elements organics carbon and oxygen isotope ratios and crystallographic features observed variations in diagenetic responses include mineralogy of the diagenetic phases rate and extent of mineral transformation distribution of foreign ions in the crystal lattice primarily sr mg and s and the number of specific processes and products along diagenetic pathways alteration is shown to be primarily controlled by the initial diagenetic susceptibility of the sample including porosimetry and structural characteristics concentrations of organic material and primary amounts of trace elements in the carbonate such as mg structural characteristics lead to initial internally fluid or rock buffered conditions with low porosity and permeability resulting in a greater effect of internal fluids and organics differences in the amount of organic content and internal fluids affect transformation rates secondary mineralogy and isotope equilibria samples with relatively high porosity high mineral transformation rates and high primary mg ca may preferentially form secondary aragonite during fast equilibration with the diagenetic environment our results suggest that the degree and nature of diagenetic alteration of aragonite materials are strongly controlled by the micro to nano scale internal architecture governing the availability and transfer of aqueous fluids results of this study provide significant implications for the interpretation of diagenetic signals in carbonate archives and have direct significance for the mechanistic understanding of carbonate diagenesis and paleo environmental conditions

structural arrangement of sedimentary organic matter nanometer scale spheroids as evidence of a microbial signature in early diagenetic processes abstract nanometer scale spheroidal objects 20 200 nm wide have been widely encountered in geological samples while their origin remains unclear first described as nannobacteria folk 1993 they are more likely associated with various bacterial debris in order to explore their potential role in degradation and preservation of organic matter om in rocks this research investigates a range of recent and fossil occurrences of various microbial imprints using electron and atomic force microscopy selected samples illustrate different stages in om cycle and diagenesis i e from production to accumulation before and after lithification more specifically effects of oxygen and light depleted conditions which occur naturally at the water sediment interface have been evaluated in microbial mats using a specifically tailored laboratory strategy recent sediments reveal a close association between a large occurrence of nanometer scale spheroids and the degree of degradation due to microbial activity wide nanometer scale spheroids appear in fossilized microbial mats and in microbial mats under experimental diagenesis these samples show that nanometer scale spheroids are associated with well preserved om e g intact bacteria this corresponds to reduced physicochemical degradation processes this observation suggests that nanometer scale spheroids i e in the size range of nannobacteria occur concomitantly with a significant microbial degradation shown by numerous organic debris e g cell walls both recent and fossil samples from anoxic settings show a close association of nanometer scale spheroids with extracellular polymeric substances eps and bacterial cell walls and not with algal cell walls or other organic compounds because they occur concomitantly with well preserved organic constituents nanometer scale spheroids indicate a reduced level of om degradation in the diagenetic processes therefore nanometer scale spheroids would be a microbial evidence of early diagenesis in sedimentary rocks

effect of igneous intrusion on the organic geochemistry of a siltstone and an oil shale horizon in the midland valley of scotland abstract the effect of rapid thermal stress on the organic geochemistry of a siltstone intruded by a quartz dolerite dyke and an oil shale intruded by an alkali dolerite sill has been studied using optical petrography rock eval pyrolysis gas chromatrography and gas chromatography mass spectrometry maturity dependent ratios derived from both the aliphatic and aromatic hydrocarbon fractions corroborate the observation of raymond and murchison 1988b that the alkali dolerite sills which were intruded soon after sedimentation had a lesser effect than the later quartz dolerite intrusions the maturity parameters based on the alkylnaphthalene and alkylphenanthrene isomer ratios are more suitable for studying maturity variations in heat affected samples than those based on the commonly used aliphatic biological markers these aromatic compounds survive to higher ranks and the isomer ratios continue changing

diagenesis of mollusc aragonite and the role of fluid reservoirs abstract biogenic carbonate minerals are widely used as archives in paleoenvironmental research providing substantial information for past depositional and diagenetic regimes however nearly all biogenic carbonates undergo post mortem diagenetic alteration to variable degrees diagenetic features are essentially caused by complex fluid solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting a common conception is that a given primary shell texture is replaced by a secondary fabric via micro scale dissolution reprecipitation reactions that may reach geochemical and or isotopic equilibrium with the diagenetic fluid here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise and the re equilibration processes can be separated in space and time more specifically attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock buffered system degradation of organics was observed via decreased sulfur concentration and decreased fluorescence in this early stage of alteration the conversion of micro scale aragonite biominerals to metastable diagenetic calcite meso crystals is documented by raman spectroscopy electron backscatter diffraction ebsd and a decrease in strontium concentrations further stabilization to larger neomorphosed calcite crystals resulted from re equilibration with external fluids and was evidenced by ebsd oxygen isotope data and further reduction of strontium spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior i e carbonate buffered versus diagenetic fluid buffered and isotope and element exchange behavior and analyzed in context to textural alteration features these data suggest that during diagenesis the evolution to more sustainable diagenetically induced fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures kinetics and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro to nano scale internal architecture governing the availability and transfer of aqueous fluids results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis

organic geochemistry and mineralogy i characterisation of organic matter associated with metal deposits abstract as geochemical appraisals of mineral regions of commercial prospectivity evolve the organic matter associated with metal rich ores has attracted greater attention petroleum basin and modern seafloor hydrothermal vent studies have suggested that organic matter can have a significant influence on the behaviour of mineralising fluids there have been many isolated reports of certain organic compositional or morphological e g pyrobitumen features showing an apparent relationships with hydrothermal fluids or minerals raising expectations that organic based parameters might be useful to mineral exploration however the understanding of organic inorganic relationships in earth systems is far from complete for example the detailed mechanics of the interaction of organics with hydrothermal fluids over geological time remain largely undefined organic geochemistry studies have traditionally involved the measurement and interpretation of the hydrocarbon composition of sedimentary rocks here we review the types of aliphatic hydrocarbons aromatic hydrocarbons and metalloporphyrins often detected from organic geochemical investigations in mineral rich regions such molecular data can be particularly diagnostic of biochemical sources and the palaeo environments at the time mineral associated organic matter was deposited sub surface trends of hydrocarbon alteration may also reflect major biogeochemical processes such as thermal maturity and biodegradation organic geochemistry data can also occasionally provide information about the nature e g origin composition temperatures and migration pathways of hydrothermal fluids and can make a contribution to holistic ore genesis models the well preserved organic matter associated with the economic here s your chance pb zn ag mine paleoproterozoic barney creek formation mcarthur basin australia and the transition metal rich early permian kupferschiefer formation germany poland have attracted significant attention a more detailed summary of the organic character of these deposits is provided to highlight the contribution organic geochemistry can make to understanding mineralisation processes most organic geochemical studies of highly mineralised regions however have not adequately addressed the significance of organic matter to mineralisation a slightly different analytical focus than traditionally used for exploration appraisal of petroleum hydrocarbons may be required to properly evaluate the significance of organic species to the mobilisation transport and deposition of ore metals the characterisation and subsequent thermodynamic modeling of organic substances and complexes within metalliferous hydrothermal systems will contribute to a better understanding of the nature and role of organic inorganic fluids or other affiliated organics in ore systems

evidence of spice related anoxia on the laurentian passive margin paired δ13c and trace element chemostratigraphy of the upper conasauga group central appalachian basin abstract this study reports data from the upper cambrian conasauga group and overlying copper ridge formation of the central appalachian region eastern u s a geochemical isotopic and petrographic analysis of core material was carried out to constrain the extent of marine anoxia and to characterize sediment fluxes on the carbonate platform and continental shelf of laurentia contemporaneous with the steptoean positive carbon isotope excursion spice a late cambrian global marine anoxic event carbonate rocks primarily dolostone record a positive 13ccarb excursion starting in the middle nolichucky formation reaching its peak 4 3 at the boundary between the maynardville and copper ridge formations strontium isotope ratios in the dolostone units are only slightly offset from the expected cambrian seawater values suggesting minimal post diagenetic disturbance of isotopic and trace element systematics selective leaching of carbonate units reveal upward decreasing trends in dissolved redox sensitive trace metals e g u ni v indicative of drawdown from regional and global marine anoxia during the peak of the late cambrian spice event

origin and significance of aromatic hydrocarbons in giant iron ore deposits of the late archean hamersley basin western australia late archean to earliest paleoproterozoic shales associated with two giant iron ore deposits in the hamersley province western australia contain traces of solvent extractable saturated and aromatic hydrocarbons the host rocks belong to the 2 5 billion years ga old mt mcrae shale and brockman iron formation hamersley group and were collected in mines near tom price and newman mt whaleback the saturated hydrocarbons in the rock extracts have the composition of highly mature gas condensates the aromatic fraction predominantly consists of unsubstituted two and three ring hydrocarbons that are demonstrably indigenous and syngenetic based on their unusual pyrolytic composition and the presence of hydrocarbons with similar attributes covalently bound to the kerogen the bitumen composition is interpreted as recording the interaction of organic matter with hydrothermal fluids or oxidizing solutions overprinted by regional low grade metamorphism the organic matter could potentially have recorded flow direction redox potential and temperature of fluids that formed the iron deposits thus a new organic geochemical approach is suggested for determining hypogene and supergene controls on iron mineralization in the hamersley province and related ore deposits

kerogen bound glycerol dialkyl tetraether lipids released by hydropyrolysis of marine sediments a bias against incorporation of sedimentary organisms over the past 10 years the membrane lipids of archaea particular glycerol dialkyl glycerol tetraethers gdgts have been recognised as significant components of marine sediments and as potentially powerful tools in palaeoclimate reconstruction however relatively little is known about their preservation and in particular their incorporation into macromolecular organic matter fractions here we examine abundances distributions and stable carbon isotope ratio 13c values of gdgts and the diether archaeol in sediments from the benguela upwelling region ocean drilling program odp site 1084 and a mediterranean sapropel s5 these are compared with distributions of biphytanes released from the kerogen solvent extracted residue fraction using hydropyrolysis hypy gdgts likely derived from pelagic crenarchaeota are amongst the most abundant extractable components in both benguela and s5 sediments but are an order of magnitude more abundant in the former moreover the benguela sediments have far greater concentrations of the acyclic gdgt than expected consistent with the presence of methane in core 1084 co occurring archaeol and variable gdgt 13c values the excess acyclic gdgt probably arises from a contribution from sedimentary methanogens hypy released biphytanes have similar abundances in benguela and sapropel sediments and represent a significant proportion of the total tetraether lipid pool at the latter site however the distributions of hypy released biphytanes especially in benguela sediments do not always match those of the free gdgts at both sites hypy released biphytane 13c values are relatively invariant consistent with a single source again likely to be pelagic crenarchaeota these observations suggest that gdgts can be incorporated into sedimentary geomacromolecules relatively rapidly with those derived from pelagic crenarchaeota more likely to be incorporated than those derived from sedimentary methanogens it is likely that this bias arises from a lack of reactive substrates i e oxygen radicals in deeper anoxic sediments consistent with an important role for oxidative cross linking in kerogen formation

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

geobiology of a palaeoecosystem with ediacara type fossils the shibantan member dengying formation south china abstract despite the importance of palaeoecosystems with ediacara type fossils for the early evolution of metazoans only little is known about the interplay of geological and biological processes in these environments the reason is that sedimentary structures biogenic structures and bio geochemical signatures e g hydrocarbon biomarkers are commonly not well preserved due to the predominance of volcanic and siliciclastic lithologies the shibantan member dengying formation south china is one of only few carbonate settings with ediacara type organisms worldwide and its lithology promises an excellent preservation of sedimentary facies and bio geochemical signatures here we provide the first comprehensive geobiological characterisation of the shibantan member in order to reconstruct the interplay between sedimentary and bio geochemical processes and to assess the microbial activities in the palaeoecosystem with ediacara type fossils facies analyses revealed that carbonate and organic matter were autochthonously formed by bio geochemical processes linked to microbial mats however the material was frequently reworked and re deposited within the same setting i e para autochthonous as evidenced by small scale hummocky cross stratification erosional contacts lenticular bedding and load casts negative ce anomalies ce ce and low v cr ratios demonstrate that molecular o 2 was present in the water column whereas characteristic ni co v v ni and v sc ratios suggest the contemporaneous presence of sub to anoxic water taken together these observations imply a temporarily stratified water body frequently mixed and ventilated by storms 13 c enrichments in the shibantan carbonates 13 c 3 29 to 3 98 vpdb together with 13 c depletions of syngenetic n alkanes cleaved from the extraction residue using catalytic hydropyrolysis hypy 13 c 31 7 to 36 3 vpdb could indicate a significant withdrawal of 12 c by primary producers that thrived within the mats at the same time sulphurised biomarkers in the bitumen and hypy pyrolysate hint at organic matter decomposition and concomitant sulphide production by sulphate reducing bacteria when oxygen was available at the sediment water interface due to mixing by storms sulphide oxidising bacteria were possibly temporarily favoured the results demonstrated that palaeoenvironmental conditions dynamically changed through a complex interplay of biogenic and abiogenic processes

reactive transport modeling an essential tool and a new research approach for the earth sciences abstract reactive transport modeling is an essential tool for the analysis of coupled physical chemical and biological processes in earth systems and has additional potential to better integrate the results from focused fundamental research on earth materials appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex earth systems reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface the description of elemental and nutrient fluxes between major earth reservoirs and in the treatment of deep earth processes such as metamorphism and magma transport active topics of research include the development of pore scale and hybrid or multiple continua models to capture the scale dependence of coupled reactive transport processes frontier research questions that are only now being addressed include the effects of chemical microenvironments coupled thermal mechanical chemical processes controls on mineral fluid reaction rates in natural media and scaling of reactive transport processes from the microscopic to pore to field scale

metasomatic vein formation by stationary fluids in carbonate xenoliths at the eastern margin of the bergell intrusion val sissone italy fluid flow is an important mechanism associated with heat and mass transport within the earth s crust the study of veins which represent channelling of fluids can thus be key in understanding these fluid movements unravelling fluid composition and origin paleo stress regimes and the history of the host rock new stable isotope data on carbonates and silicates have been combined with phase petrology mass balance and field observations to evaluate the formation mechanism of metasomatic reaction veins in dolomitic xenoliths in the bergell tonalites val sissone italy multiple generations of extensional veins can be followed from the contact zone between the dolomites and the intrusion to a few meters within either the tonalites with the epidote quartz veins or within the dolomites where they terminate each type of vein contains a central zone which is formed by open fracture crystallization this central fracture is framed by relatively symmetric replacement zones where the original dolomite reacted to form either forsterite diopside tremolite or talc all accompanied by calcite in either a succession of reaction zones or in simpler bi mineral silicate calcite veins the 18o and 13c values across the veins allow temperatures to be estimated from different mineral pairs silicate calcite and which confirm vein formation along a retrograde cooling path of the intrusion at least four different fluid infiltration events are required the first one around 555 c to form the forsterite calcite veins followed by the epidote quartz veins at temperatures around 430 c then the tremolite calcite veins at around 390 c and finally the talc calcite veins at around 140 c the shape of the 18o and 13c profiles which are flat across the central part and the replacement zones of the veins buffered by the intrusion change substantially over short distances both of these isotope profiles overlap with the equally sharp mineralogical front between the veins and the unreacted dolomites these profiles are interpreted to be the result of an isotopic exchange mechanism driven by dissolution and re precipitation reactions all veins are oriented perpendicular to the contact with the intrusive body except for the late talc veins elevated fluid pressures above the confining pressures caused by the regional and intrusion emplacement stress field are suggested to be responsible for the initial fracturing of the carbonates and intrusive rocks the contact zones between the tonalites and carbonates likely served as fluid conduits where fluids accumulated and the pressure built up until hydrofracturing occurred we propose that the veins formed through episodic pulses of highly reactive fluids that remained stationary during reaction rather than a system where fluids flushed through the veins based on the xco2 constrained mass balance the formation of the veins would only require a relatively small amount of fluid which could potentially originate from the intrusive rocks in vicinity of the xenoliths veining is not ubiquitous around the bergell intrusion suggesting that it only may have been a localized event and thus there is no need to involve a larger convective hydrothermal system for their formation

dolomite selectivity and mimic replacement dolomite can 1 selectively replace certain components of a rock such as coralline algae and 2 mimically replace certain fossils such as echinoid fragments we dolomitized coralline alga echinoid foram pelecypod halimeda gastropod and coral fragments at 250 c in two molar cacl 2 mgcl 2 solutions and found that coralline algae echinoids halimedae and forams dolomitized faster than the other fossils echinoids and forams were mimically replaced whereas pelecypods gastropods and corals were replaced by dolomite which destroyed the original skeletal architecture artificial conversion of high mg calcite coralline algae and echinoid fragments to low mg calcite prior to dolomitization had no discernible effect on the rate of dolomitization or the fabric of the dolomitized fossils on the basis of these results and petrographic analyses of natural low temperature dolomites we conclude that mimic replacement and fossil selective replacement is more likely to be affected by the crystal size than by mineralogy

sterane and triterpane biomarkers in the precambrian nonesuch formation north american midcontinent rift abstract molecular fossils and organic matter from microbial communities living around 1 1 ga ago are well preserved in the nonesuch formation of the us midcontinent rift system although some compound classes of biological markers have been recognized repeatedly in previous studies of nonesuch rocks and associated petroleums steranes and triterpanes have not been detected failure to detect these compounds was due primarily to instrumental limitations and the relatively high thermal maturity of samples collected within zones of copper mineralization our analyses of nonesuch samples from the white pine mine and nonmineralized localities in michigan and wisconsin reveal suites of c 26 c 30 steranes and c 27 c 35 pentacyclic triterpanes similar to those found in phanerozoic oils and sedimentary rocks of marine origin stereoisomeric distributions in the nonesuch samples are consistent with moderate thermal maturity except in the mineralized zone near white pine

biomarkers of black shales formed by microbial mats late mesoproterozoic 1 1 ga taoudeni basin mauritania a b s t r a c t hydrocarbon biomarkers in late mesoproterozoic black shales from the taoudeni basin mauritania northwestern africa were analysed for palaeoenvironmental reconstruction within the atar group touirist formation shales showed the highest content of organic carbon 20 at very low maturity rc less than 0 6 microfacies and biomarker data indicate a shallow marine high productivity low oxygen setting with benthic mats as key players for the accumulation of organic matter both high c s ratios and the occurrence of rearranged hopanes indicate that euxinic conditions were not prevalent likely reflecting the shallow depositional environment steranes were not observed indicating only a minor importance of modern eukaryotic algae although low in abundance a palynological survey however reveals the presence of simple ornamented acritarchs input of highly aromatic biopolymers typical of mesoproterozoic acritarchs or microbial exopolymeric substances may be the origin of an unusually high aromaticity observed for the biomarker extracts high amounts of hopanes suggest that the benthic mats were dominated by cyano bacteria furthermore the presence of 2 3 6 trimethyl aryl isoprenoids points at contributions of organic matter from anoxygenic phototrophic bacteria however low concentrations of these compounds argue against major photic zone anoxia in the overlying water column and rather suggest a role of anoxygenic phototrophs as part of the benthic microbial mat community the high abundances of hopanes in these samples suggest that nitrogen limited conditions may have been common in the taoudeni basin at 1 1 ga these results are consistent with ideas of widespread nitrogen deficiency that emerged in mesoproterozoic oceans due to high denitrification rates in anoxic deep waters cyano and other phototrophic bacteria as well as a limited number of acritarch species were able to cope with such conditions much better than modern algae with their higher nitrogen and trace metal demands 2011 elsevier b v all rights reserved

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

micro ablation a new technique to remove drilling fluids and other contaminants from fragmented and fissile rock material abstract we present a new methodology micro ablation that can remove sub millimeter layers of contaminated surfaces from pieces of sedimentary rock that are too small or irregularly shaped to be trimmed using a saw recognizing traces of contaminants is crucial when analyzing samples with low indigenous bitumen content or interpreting trace biomarker components in hydrocarbon rich samples quantitative analysis of individual hydrocarbons in the interior and exterior of these rock fragments can then help to distinguish between autochthonous and allochthonous hydrocarbons

molecular evidence for life in the 3 5 billion year old warrawoona chert article i nfo the biological origin of organic matter in the oldest siliceous sediments cherts is still debated to address this issue the insoluble organic matter kerogen was isolated from a chert of the warrawoona group the chemical structure of the kerogen was investigated through a combination of analytical techniques including solid state 13 c nuclear magnetic resonance and pyrolysis although dominated by aromatic hydrocarbons the pyrolysate comprises a homologous series of long chain aliphatic hydrocarbons characterized by odd over even carbon number predominance this distribution is only consistent with a biological origin as kerogen must be contemporaneous of the solidification of the chert this observation should be regarded as an evidence for the presence of life on earth 3 5 by ago

calcium isotopes in the global biogeochemical ca cycle implications for development of a ca isotope proxy abstract at the earth s surface calcium ca is a critical element at a variety of scales it is soluble in natural waters a biological nutrient and a major constituent of the dominant mineral sink for carbon in the ocean caco3 there is a 4 variation in the ca isotopic composition 44ca 40ca expressed as 44ca of various ca reservoirs on earth suggesting ca isotopes as a promising tracer of ca cycling in both the present and the past fifteen years of high precision ca isotope measurements has revealed much about the behavior of ca isotopes in the earth surface environment but there remain fundamental questions concerning how ca isotopes are used to elucidate the marine and terrestrial ca cycles the current work presents a data compilation of over 70 published ca isotope studies totaling over 2600 measurements presented on a common delta scale that includes data on rivers and groundwater dust soils and soil pore fluids vegetation rainwater silicate minerals rocks and authigenic marine minerals carbonates sulfates and phosphates both modern and ancient the data compilation suggests that 1 there is a significant difference between carbonate 0 60 and silicate 44ca 0 94 2 riverine 44ca 0 88 does not simply reflect the compiled carbonate 44ca and 3 terrestrial vegetation exhibits the largest range of ca isotopic compositions 3 5 in the terrestrial setting we discuss these observations in the context of the global ca cycle exploring the extent to which seawater 44ca variability is feasible and how we can achieve accurate reconstructions of seawater 44ca over geologic time scales the current study presents simple mass balance models that quantify the leverage of inputs to change the ca isotopic composition of the ocean as this directly impacts the manner in which ca isotopes are interpreted although ca fractionates isotopically in the modern system during continental cycling the 44ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite in the terrestrial realm plants exhibit a wide 44ca range and there is evidence that ca fluxes via biomass degradation are significant at the catchment scale we therefore assess the ability of the continental biosphere to influence riverine and consequently seawater 44ca a steady state biosphere has little leverage to alter riverine 44ca except in cases where the 44ca of the recycling flux is isotopically distinct from the 44ca of the uptake flux a non steady state biosphere can substantially impact both soil and riverine 44ca driving exchangeable ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux based on estimates of the size of the global biosphere 1 5 1015 mol ca we suggest a decaying biosphere has the potential to impact riverine 44ca by tenths of a permil over time scales in the marine realm we evaluate the effect of a variable fractionation factor accompanying global removal of ca from the ocean on seawater 44ca and suggest methods by which such a mechanism can be recognized in the rock record experimental data suggest that there is considerable leverage