aragonite relic preservation in jurassic calcite replaced bivalves shells of the aragonite bivalve neomiodon great estuarine group jurassic scotland replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure the presence of these relics in such old altered shells as well as the high sr content of the replacement calcite indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events but rather a rapid one step process aragonite relics once encased in neomorphic spar will survive as unequivocal evidence of original aragonite mineralogy barring total remobilization of the enclosing stable calcite a generally unlikely event the retention of this residual aragonite and high sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration recrystallization invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or as in the case of our neomiodon material deeper burial environments pseudopleochroism of the replaced neomiodon shells appears to be due to organic largely graphitic relics not to the aragonite relics

identification and diagenesis of a phylloid alga archaeolithophyllum from the pennsylvanian providence limestone western kentucky abstract phylloid algae in the pennsylvanian providence limestone desmoinesian in western kentucky are typical in appearance thin arcuate leaflike forms with a calcite mosaic interior cathodoluminescence and backscattered electron imaging reveal well preserved skeletal structure and their identity as archaeolithophyllum sp johnson 1956 and a missouriensum johnson 1956 thin skeletal walls are composed of nonluminescent neomorphic calcite polygonal hypothallial and perithallial cells in the skeletons are filled by luminescent calcite cement chemical staining also distinguishes cell morphology when ferroan calcite cement is present in skeletal pores in contrast to the nonferroan calcite forming skeletal walls hypothallial cells in archaeolithophyllum are 25 m 50 m and are commonly arranged in arcuate layers skeletal walls 5 m thick are composed of calcite and lack distinctive relict features although some are microporous and contain fine disseminated pyrite some thalli contain large irregularly shaped hypothallial cells 50 100 m partitioned by skeletal walls these are interpreted as poorly calcified thallus fragments neomorphic calcite of the skeletons contains high concentrations of sr average 3702 ppm range 339 10 549 ppm strongly suggesting that the original archaeolithophyllum skeleton was aragonitic archaeolithophyllum shares morphological structural and compositional characteristics with holocene peyssonnelid algae this evidence substantiates earlier hypotheses that archaeolithophyllum may be an ancestral peyssonnelid alga calcite cements filling skeletal cells include a bright yellow cl phase and two subsequent phases of dull red and dull yellow luminescence mg mn and fe are enriched in these calcites reflecting reducing diagenetic fluids particularly enriched in fe cementation of skeletons by ferroan calcite during early diagenesis appears to have protected skeletons from dissolution and promoted slow rates of neomorphism conditions of a nearly closed system are constrained by the high retention of sr in the skeletal calcite

cyclic sedimentation in the southern alpine rhaetic the importance of climate and eustasy in controlling platform basin interactions uppermost triassic rhaetic facies as developed in the southern alpine region of northern italy were deposited in a rapidly subsiding fault dissected trough the lombardy basin bounded by carbonate platforms the main part of the rhaetic succession consists of 10 m scale asymmetric cycles each divided into three parts a lower shale portion a central rhythmic portion consisting of repeated marl limestone couplets the limestone parts of which thicken upward and an upper wholly carbonate unit a study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone marl couplets of the central rhythmic member together constituting a compound cyclic form are fundamentally depositional in nature it is suggested that this compound cyclicity resulted from the superposition of a low frequency approximately 100 000 year periodicity asymmetric carbonate mud signal with a higher frequency terrigenous mud signal field petrographic and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin having been derived as relatively pure aragonitic mud from adjacent carbonate platforms it is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea level variations occurred a model is presented in which the basinward export of carbonate was negligible in the deepening phase increased to a maximum during shallowing and was finally halted by the emergence of large platform flats in contrast the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated fluctuations of a shorter period than those predicted by the milankovitch theory affected hinterland precipitation and runoff particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal

geodynamic control on carbonate diagenesis petrographic and isotopic investigation of the upper jurassic formations of the paris basin france abstract the oxfordian and kimmeridgian carbonates of the eastern edge of the paris basin display poor reservoir properties stable isotope investigation of these non recrystallized cements through both microdrilling sampling and in situ sims secondary ion mass spectrometry analysis shows that they originated from slightly buffered meteoric fluids 3 smow to 8 smow flowing through the malm carbonate aquifers of the eastern edge of the paris basin after a review of the post upper jurassic history of the paris basin it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin s geodynamic setting and notably with the lcu late cimmerian unconformity major event which corresponds to the uplift of the london brabant massif denudation of the oxfordian and kimmeridgian carbonates on the southern flank of the london brabant massif to the north of the study area then lead to lateral recharging of the associated aquifers with meteoric fluids this scenario is similar to the one proposed to explain cementations of the jurassic carbonates in the wessex weald basins which emphasizes the main role played by the cretaceous unconformities over carbonate diagenesis at the scale of the northwestern tethys margin

a review of the nature and origin of limestone microporosity abstract limestone microporosity is ubiquitous and extensively developed in most phanerozoic limestones from an economic perspective microporosity is important because it contributes substantially to the carbonate pore system which can host significant volumes of water and hydrocarbons therefore determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations reservoir characterization and fluid flow simulations from an academic standpoint microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments a fundamental process in carbonate diagenesis many types of micropores contribute to what has been referred to as microporosity but the vast majority is hosted among low magnesium calcite lmc microcrystals that are present in limestone matrix and allochems geochemical textural and mineralogical data from natural settings and laboratory experiments indicate that lmc microcrystals are diagenetic in origin more specifically these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high magnesium calcite hmc minerals and precipitation of lmc microcrystal cements the stabilization process is inferred to take place in the meteoric marine and burial diagenetic realms although it has not been directly observed carbon and oxygen isotopes as well as trace element data suggest that lmc microcrystals form during burial diagenesis in marine like fluids evidence suggests that porosity is not generated during this dissolution precipitation process but rather inherited from the precursor sediments the final arrangement of the micropores in a limestone however depends on the precise diagenetic pathway lmc microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size the three main textural classes granular framework fitted mosaic and clustered have been recognized across a wide range of ages depositional settings burial depths and precursor types and are characterized by distinct petrophysical properties such as porosity permeability and pore throat size observations from modern sediments also support the hypothesis that lmc microcrystals develop from aragonite and hmc dominated lime mud the origin of lime mud has been extensively studied but still highly debated of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the phanerozoic microporous limestones comprised of lmc microcrystals are equally abundant during times of aragonite seas and calcite seas which suggests that no special mineral precursor is required microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents unlike shallow marine limestones deep marine sediments start out as mostly lmc therefore mineralogical stabilization is not a significant process in chalk diagenesis

concretions of the valtos sandstone formation of skye geochemical indicators of palaeo hydrology the calcite concretions of the jurassic valtos sandstone formation record the origin of the porewaters within which they grew and the approximate flow rates the concretions grew within meteoric porewaters part of a middle or upper jurassic meteoric water lens extending into the sea of the hebrides basin from the then hebridean landmass modal porefluid flow rates were low resulting in an approximately spherical concretion geometry the concretions are zoned due to episodic recharge of the aquifer which is tentatively related to seismic pumping the sandstone was open with respect to mg fe mn and sr but closed with respect to ca the sandbody was rock dominated with respect to ca sr c and mn which were derived from within the sandbody by internal redistribution but water dominated for fe mg and o the valtos concretions grew by the diagenetic redistribution of aragonitic shells and comprise only a single phase of ferroan calcite feco 3 0 89 2 7 mole with a small range of carbon compositions 13 c 0 25 to 0 97 pdb the cements have a large range of oxygen isotopic compositions 18 o 8 3 to 18 2 pdb interpreted to represent changes in porewater composition not formation temperature cement crystal size increases radially reflecting decreasing degrees of supersaturation during growth the cements have only a small range of 9bulk9 fe and mg concentrations as determined by icps but a large range of concentrations determined by microprobe this pattern is explained by disequilibrium trace element partitioning associated with sector zoning within the cement

chemical and isotopic evidence for hydrogeochemical processes occurring in the lincolnshire limestone over 150 groundwater samples from the lincolnshire limestone have been analysed for ph major ions and 13c ratios where possible field eh and iodide concentrations were measured and methane concentrations were determined for 12 samples stable isotope ratios were determined for soil and rock carbonate samples a system of zonation allows the division of hydrogeochemical processes occurring in the aquifer the use of hydrochemical and isotope data in modelling exercises enables the re evaluation and possible enhancement of the understanding of hydrogeochemical processes the carbonate chemistry of outcrop groundwaters is explained by calcite saturation being achieved under open system conditions in the soil zone 13c ratios in the range 15 99 to 10 57 may be generated from a stoichiometric reaction with possible additional partial and or simultaneous exchange with soil co2 or carbonate the isotopic composition of soil carbonate shows the effects of precipitation from soil waters the incongruent dissolution of primary depositional limestone carbonate results in increasing magnesium and strontium concentrations and increasing 13c ratios for the groundwaters with flow down the hydraulic gradient as a result of incongruent dissolution secondary calcite may be precipitated onto fissure surfaces significant nitrate and sulphate reduction in non saline groundwaters is not supported by the results of hydrochemical and isotope modelling exercises however sulphate reduction and methane fermentation may be affecting the isotopic and chemical compositions of saline groundwaters sodium calcium ion exchange leads to limited calcite dissolution deep in the aquifer but the evolution of these groundwaters is confused by the uncertain effects of oxidation of organic carbon and mixing with a saline end member solution

skeletal aragonite neomorphism quantitative modelling of a two water diagenetic system abstract the neomorphism of skeletal aragonite occurs by the simultaneous dissolution of aragonite and precipitation of calcite along thin solution films which may have an associated zone of partly dissolved aragonite chalky zone it has been proposed that pore waters at neomorphic calcite aragonite contact zones neomorphic fronts may be partly isolated from bulk pore waters and thus retain primary geochemical signatures to varying degrees iterative computer programs were developed that simulate aragonite neomorphism in two water diagenetic systems using stable isotope and trace element data neomorphism of aragonitic mollusks from the purbeck group of southern england late jurassic to early cretaceous was associated with a decrease in strontium concentration from approximately 5100 ppm to 500 ppm modelling results indicate a high bulk water rock ratio and minimal if any isolation of the neomorphic front waters for the purbeck group mollusks and also for some pleistocene corals partial isolation of neomorphic front waters from bulk pore waters is uncommon based on published data on the geochemistry of neomorphic calcites however partial isolation does occur in some thick low porosity shells such as the gastropod strombus gigas pleistocene process based modelling of fluid rock interaction provides a means for better interpreting trace element and stable isotope data in diagenetic carbonates sensitivity analysis results indicate that water rock ratios cannot be determined at much better than order of magnitude accuracies because of uncertainties over initial pore water composition fractionation factors and distribution coefficients

geochemical indicators of depositional and early diagenetic facies in holocene carbonate muds and their preservation potential during stabilisation holocene carbonate muds 63 m fraction and macro organic matter were collected from marine subtidal mudbanks and supralittoral mud islands of florida bay and freshwater marshes of the florida everglades u s a geochemistry sedimentology and mineralogy were used to determine whether carbonate mud formation in specific marine and freshwater environments was geochemically distinct and to assess the importance of early diagenetic reactions on mud chemistry and mineralogy significant variations were found in the relative amounts of carbonate minerals not just between the marine and freshwater environments but also within florida bay in particular mg calcite is enriched in crane key muds by 10 relative to nearby cross bank the mg calcite appears to precipitate in the surface living cyanobacterial mats of the key and seems to be linked to mg chelation by the cyanobacterial organic matter this mg calcite has a distinct 13c around 4 6 lighter than mg calcite derived from thalassia epibionts in the cross bank muds dolomite was found just below the cyanobacterial mats forming in the only part of the core where reactive organic carbon has been completely removed by sulphate reducing bacteria the 13c values of this dolomite at least 4 are consistent with previously published 13c values of dissolved inorganic carbon in pore waters at this horizon which support dolomite formation in situ mg supply for dolomite formation may be from dissolution of the overlying cyanobacterial mat mg calcite the various types of organic matter have distinctive 13c values ranging from 25 mangrove and sawgrass to 10 cyanobacterial mats and 8 thalassia seagrass values intermediate between these end members record mixing of the organic matter types which shows particularly the time when the bank muds became dominated by thalassia organic matter i e were relatively removed from mangrove vegetation five holocene mud suites bank key cyanobacterial mats mangrove swamp and freshwater everglades swamp show distinct chemical signatures in their mean 13c 18o mgco3 and sr values na distinguishes between seawater and non seawater influenced environments but fe mn and p are mainly controlled by non carbonate materials comparison with published stable isotope and elemental data for pleistocene limestones in the south florida area suggests that only the facies diagnostic signatures of 13c and mg will be resolvable once the muds have been stabilised to micrite alteration of the 18o and sr values during stabilisation will probably be more severe to the extent that original facies variations are destroyed

87sr 86sr and sr ca investigation of jurassic mollusks from scotland implications for paleosalinities and the sr ca ratio of seawater measurements of 87 sr 86 sr sr ca 18 o and 13 c were performed on molluskan fos sils from the great estuarine group scot land to deduce paleosalinity patterns and hydrodynamic environments in this classic jurassic marginal marine succession fos sils are found as winnowed accumulations in shell beds that bear low diversity assem blages consistent with deposition in a la goonal paleoenvironment previous studies have concluded that estuarine mixing of seawater and freshwater took place based on correlated trends in 18 o and 13 c most notably in the mussel praemytilus strathair densis from the kildonnan member this interpretation is shown to be incorrect based on 87 sr o 13 c and the sr ca ratio this is consistent with a lagoonal hydrology controlled more by seasonal changes in evaporation precipitation and flooding than by direct inputs of seawater or river ine water transported over large distances lagoons containing the oyster praeexogyra in the duntulm formation have similar in ferred o and c isotope characteristics but based on their 87 sr 86 sr ratios they have a higher proportion of seawater than the praemytilus lagoons of the kildonnan member the data thus far suggest that many of the lagoonal environments in the great es tuarine group were nearly hydrologically closed at least over the interval represent ing the molluskan invasion of these habi tats rendering o and c isotopes unreliable proxies of paleosalinity because strontium isotopes are not directly affected by evap oration of the lagoons knowledge of marine and freshwater 87 sr 86 sr ratios along with estimates of sr concentration are used to calculate the proportion of seawater in the brackish water mixture unio is nearly al ways the most radiogenic taxon compared to other genera in closely adjacent sedimen tary strata confirming its freshwater affin ity the purported marine brackish bivalve tancredia yields a low paleosalinity of 4 only slightly more saline than unio and praemytilus at 1 3 the oyster praeexo gyra hebridica yields paleosalinities between 2 and 26 attesting to the wide salinity tolerance of this species thus far only neomiodon from the dun tulm and staffin bay formations show co variant relationships between 87 sr 86 sr and ca sr ratio that can be reconciled with a dynamic response to seawater freshwater mixing the slopes of these trends converge at 0 7068 which is close to the early cal lovian seawater composition of 0 7069 us ing values of dsr from modern aragonite se creting mollusks the sr ca ratio of middle jurassic seawater is constrained from the best correlated mixing line to be 5 4 0 6 millimoles mole in contrast sr ca ratios in the great estuarine group lagoons and continental watershed range from 5 4 to 19 0 these are much higher than the av erage value of 2 3 for modern rivers but typical of sr ca ratios in rivers draining arid climate regions the semirestricted hy drodynamic regime inferred from the high seawater like 18 o values and high but uni form sr ca ratios in many of the great es tuarine group mollusks is consistent with a seasonally dry mediterranean style climate for the middle jurassic of britain

microbial communities and their primary to early diagenetic mineral phases the record from neoproterozoic microbialites of qarn alam oman

buried vertisols in lacustrine facies of the pliocene fort hancock formation hueco bolson west texas and chihuahua mexico buried vertisols which are commonly preserved in smectite rich clay and sandy clay facies of the upper tertiary fort hancock formation are products of the late tertiary depositional and paleoclimatic environment in the hueco bolson west texas united states and chihuahua mexico fort hancock clayey facies are in part laminated and contain abundant desiccation cracks and locally gypsum beds suggesting deposition in an ephemeral or playa lake buried vertisols apparently resulted from repeated shrink swell cycles due to drying and wetting of expansive clays in the ephemeral lake basin lacustrine facies are interbedded with gravel sand and sandy mud facies that were likely deposited as alluvial fans and fan deltas at the lake margin calcic soil horizons which form mostly in arid to subhumid modern climates and consist of caco 3 nodules and filaments are present in most buried vertisols of the fort hancock formation collectively buried vertisols with calcic soil horizons and ephemeral lake and alluvial fan deposits suggest that the fort hancock formation accumulated in an arid to semiarid climate

diagenesis of late miocene micritic lacustrine carbonates madrid basin spain mud grade lacustrine limestones of miocene age from the madrid basin spain reveal varied and complex diagenetic histories microporous micrites occur as well as micrites with anhedral mosaics the micrites have recrystallized from a metastable precursor in part under sub oxic meteoric diagenetic conditions probably before extensive karstification in the pliocene the absence of aragonite relics their mainly micritic microfabric low sr content and covariant trend of sr and mg suggest that the micrites formed from mainly high magnesian calcite muds lacustrine micrites can be compared to marine forms and criteria used to assess the composition of the marine precursor muds can be applied to lacustrine limestones to complement other techniques to identify original lake water compositions

two phase diagenetic alteration of carbonate matrix middle ordovician limestones taiyuan city area china both pseudospar and micrite are present in carbonate matrix of middle ordovician wackestones in the taiyuan city area china under the scanning electron microscope pseudospar crystals display unpitted surfaces and an absence of aragonite relics in polished and slightly etched sections micritic crystals show a similar feature with slightly pitted surfaces and absence of aragonite relics this possibly indicates that both pseudospar and micrite have a similar original mineralogy i e as calcites this is further supported by their low sr contents 417 ppm the geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix micrite was possibly formed in near surface meteoric environments but pseudospar which is characterized by a population of more depleted 18o values 9 3 to 10 2 pdb than that of micrite 7 4 to 8 7 pdb is explained to have formed during shallow burial diagenesis because its 18o composition is too light to be accounted for by near surface meteoric origin the lower sr contents 417 ppm of both pseudospar and micrite in conjunction with their more depleted 18o values 7 4 to 10 2 pdb are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites the more negative and wider range of 13c values of pseudospar and micrite 0 3 to 4 7 in comparison with those of other coeval meteoric calcites was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis although the contents of fe and mn between pseudospar and micrine overlap widely an inverse relationship exists between the concentrations of fe and mn and the 13c values in both pseudospar and micrite this can be accounted for by variable redox by variable redox conditions during diagenesis the 18o values of void fill sparry calcite cements in micrite 8 2 to 8 6 pdb fall into the category of near surface meteoric diagenesis possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations so that only minor amount of pseudospars were formed during shallow burial diagenesis by recrystallization

effect of aragonite to calcite transformation on the geochemistry and dating accuracy of speleothems an example from castanar cave spain aragonite speleothems are increasingly being used as high resolution climate proxies however aragonite is unstable and susceptible to diagenetic transformation into calcite leading to mineralogical textural and geochemical alterations that have not been fully investigated to provide some insights to these modifications this study combines stable isotope geochemistry and u series dating with petrological observations and empa elemental analyses to characterize a stalagmite from castanar cave caceres spain that shows primary aragonite diagenetically altered and with reverse ages the diagenetic processes include recrystallization to calcite micritization and dissolution characterization and interpretation of the chemistry and nature of the waters from which the aragonite precipitated and of the ones that subsequently formed the calcite indicates that both polymorphs formed from the same fluids these fluids hydrochemically evolved from an aragonite mode to a calcite mode along a diagenetic path within the sample the results show that during transformation the system stayed close in one of the stages and open in the other with the same textural result reverse ages are found either in the recrystallized areas or in the primary aragonite caused by two different u remobilization mechanisms u leaching due to flowing waters on the surface and u loss during the transformation process these results highlight the complex behavior of speleothems during diagenesis and the impact on the geochemical information of the primary and secondary phases and thus the importance of petrological and geochemical characterization of speleothems for paleoclimatic studies

the mechanism of skeletal aragonite neomorphism evidence from neomorphosed mollusks from the upper purbeck formation late jurassic early cretaceous southern england abstract the trace element geochemistry and microtextures of neomorphosed mollusks from the upper purbeck formation late jurassic early cretaceous of southern england suggest that calcite replacement of aragonite occurs by a force of crystallization driven mechanism similar to that of non polymorphic skeletal carbonate replacement reactions such as silicification the purbeck neomorphic calcites are substantially enriched in mg mn and fe and depleted in sr compared to skeletal aragonite indicating that the neomorphic reaction zones were not chemically isolated from the bulk pore waters to a significant degree neomorphic calcite microtextures are fundamentally similar to the replacement microtextures of silicified and celestite replaced fossils in terms of the degree and style of preservation of traces of skeletal microtextures in authigenic crystals and the relationships of authigenic crystal morphology and crystallographic orientation to shell microtexture the neomorphic replacement of aragonite by calcite differs from most non polymorphic replacement reactions in that the calcite cannibalizes aragonite decreases in the intraskeletal pore water calcium carbonate ion activity product caused by calcite precipitation results in aragonite dissolution adjacent to aragonite calcite contacts i e chalkification in some fossils chalkification does not occur in non polymorphic replacements of skeletal carbonate

understanding a bimineralic bryozoan skeletal structure and carbonate mineralogy of odontionella cyclops foveolariidae cheilostomata bryozoa in new zealand abstract skeletal carbonate mineralogy of the bryozoan odontionella cyclops busk 1854 family foveolariidae is extremely variable with calcite aragonite ratio ranging from 27 to 100 wt calcite mean 57 wt calcite sd 15 n 118 and mg content in calcite varying from 3 6 to 8 8 wt mgco3 mean 6 2 wt mgco3 sd 1 1 n 118 this study examines the sources of this wide variability and the possible effects of ocean acidification on bimineral invertebrates variation in calcite aragonite ratio in o cyclops is neither environmental nor related to colonial growth form but appears to be astogenetic primary calcification of the zooecial box is all calcite followed by progressive construction of a secondary aragonitic superstructure which includes avicularia consequently young parts of the colony are dominated by calcite with increasing amounts of aragonite with age very old parts of the colony may have the aragonite eroded or chipped away to become again entirely calcitic in contrast with many other bryozoans that are entirely calcitic or mainly aragonitic this bipartite structure may result in increased vulnerability to ocean acidification given the southern temperate shelf to slope distribution of this species o cyclops and others like it will begin to be subjected to decreasing ph in only a few decades the consequence could be a modern sediment assemblage similar to a diagenetically altered fossil assemblage missing aragonitic skeletal parts and species

understanding biomineralization in the fossil record abstract biomineralization the formation of minerals by organisms is a key aspect in the understanding of the fossil record knowing how biominerals form and their properties is important in the correct use of fossils in geochemistry the understanding of evolution and in the interpretation of how geological events have influenced the fossil record throughout the phanerozoic the focus of this contribution rather than a conventional review on the status of this research field is on the importance of highlighting the traditional link between paleontology and biomineralization

hydrochemistry of carbonate terrains in alpine glacial settings nearly 200 analyses of meltwaters ice and snow from three alpine glacial sites in carbonate terrain are summarized and discussed in terms of sources of solutes and kinetic controls on the progress of weathering reactions most data derive from the swiss glacier de tsanfleuron which is based on cretaceous and tertiary pure and impure limestones two other sites marmolada italian dolomites and the saskatchewan glacier alberta are based on a mixed calcitic dolomitic substrate most solutes originate from carbonate dissolution moreover where pyrite is present its oxidation supplies significant sulphate and forces more dissolution of carbonate the ratios sr2 ca2 and mg2 ca2 are much higher in tsanfleuron melt waters than local bedrock a phenomenon that can be reproduced in the laboratory at small percentages of dissolution these anomalous ratios are attributed to incongruent dissolution of traces of the metastable carbonates mg calcite and aragonite these phases also provide na to solution k is argued to originate mainly by ion exchange on clays with solute ca2 quartz and very minor feldspar dissolution are also inferred locally enhanced input from atmospheric sources is recognized by high cl and associated na the progress of weathering reactions has been evaluated by the trends in the data computer modelling and some simple laboratory experiments the most dilute samples show a trend towards removal of co2 to low partial pressures c 10 5 5 atmospheres reflecting initially rapid carbonate dissolution and relatively slow dissolution of gaseous co2 later addition of atmospheric co2 or acid from pyrite oxidation allows further carbonate dissolution but solutions show a wide range of saturations and co2 pressures as high as 10 2 2 where pyrite oxidation is important in a carbonate terrain measurement of electroconductivity corrected to 25 c and alkalinity in the field allows the following preliminary deductions where meq stands for milliequivalents formula presented where s is the minimum meq ca2 mg2 produced by simple dissolution of carbonate unconnected with pyrite oxidation as with any proxy method these deductions do not remove the need for chemical analysis of waters in a given study area copyright 1994 john wiley sons ltd

δ18o δ13c trace elements and ree in situ measurements coupled with u pb ages to reconstruct the diagenesis of upper triassic atoll type carbonates from the panthalassa ocean abstract owing to their isolated oceanic setting atoll type carbonates are well suited for documenting carbonate deposition and diagenesis in oceanic environments away from continental influence the atoll type dalnegorsk limestone taukha terrane russian far east deposited in the gigantic but poorly documented panthalassa ocean preserves a complete record of the diagenetic evolution of an upper triassic system out of the tethyan domain to study the diagenesis of this carbonate system we developed a novel analytical workflow combining cathodoluminescence petrography with high resolution analyses of environmental proxies in calcitic cements 18o 13c reey trace and minor major elements and in situ u pb dating of calcite cements to precisely reconstruct the chronology of the diagenetic events we combined these lines of evidence to establish a model of atoll evolution from deposition to dismantling based on 10 identified diagenetic episodes the dalnegorsk limestone records emergence at the norian rhaetian transition marked by meteoric and evaporitic cements followed by dismantling of the atoll edges after drawning in the early jurassic neomorphism of calcitic shells occurred at the onset of calcitic sea conditions during the toarcian bajocian the limestone was thoroughly cemented during the middle late jurassic and accreted within the taukha terrane during the late jurassic early cretaceous accretion resulted in fracturing brecciation and recrystallisation of the dalnegorsk limestone this model is potentially applicable to any similar atoll system irrespective of age the evidence presented here extends our knowledge of late triassic environments in the panthalassa ocean and more generally our understandingg of mid oceanic limestone formation and evolution

dna and fluorescein tracer tests to study the recharge groundwater flowpath and hydraulic contact of aquifers in the umbria marche limestone ridge central apennines italy the purpose of this paper is to highlight the hydrogeological contact among aquifers in karst and fissured systems and study the different flowpaths present in said aquifers given the complex hydrogeological and tectonics settings detailed surveys in the umbria marche limestone ridges and tracer field tests were used to achieve this goal dna and fluorescent tracers were injected into a sinkhole and thereafter recovered at different points allowing both for the identification of contact among aquifers as well as the main and secondary directions of groundwater the tracer tests have shown that the majority of the groundwater flow is directed towards the axis of the tectonic structures emerging in more depressed areas in addition they have confirmed the influence of karstification on the recharge which allows for a more detailed characterisation of water circulation in the unsaturated zone of the maiolica and the calcare massiccio limestone aquifers thus offering confirmation of the hydrogeological survey results and providing additional information that was previously unknown furthermore the dna tracer provided good results in the field especially in the karst systems and fissured rocks thus confirming its appropriacy for flowpath investigations and vulnerability analyses of springs in particular dna is suitable for very large amounts of groundwater owing to its very low detection limit further investigations and studies are however required to assess its validity also for the evaluation of hydrogeological parameters

shell beaches to beachrock diagenetic stable isotopic trends abstract modern shell accumulations on padre island texas gulf coast are composed of pristine aragonite whereas laterally adjacent pleistocene beachrock on the mainland shores of laguna madre are made up of the same taxa but display a range of mixtures of aragonite and diagenetic low mg calcite cements and pedogenic crusts associated with the beachrock are 100 low mg calcite stable isotopic values for shells composed of 0 15 20 45 75 and 90 calcite display high correlations between the 18o values and mineralogy r2 0 95 and 13c values and mineralogy r2 0 87 the highly correlated changes in the stable isotopic compositions and mineralogy of the skeletal material are also evident in the data from the analyses of the pure calcite cements and pedogenic crusts changes in stable isotopic composition of the shells are the result of the early diagenetic alteration of the original aragonitic composition to low mg calcite

an oxygen minimum palaeoceanographic signal from early toarcian cavity fills moderately deep marine toarcian sedimentary rocks of the medianes nappe alpine front ranges switzerland comprise calcarenites and lime mudstones they were deposited under storm and fairweather conditions in the dysaerobic zone near the storm wave base these remnants of the northern margin of the tethyan ocean show facies where primary sediment 9deep water9 stromatolitic micrite was reworked during early diagenesis stromatactis like cavities were formed by carbonate dissolution near the sediment water interface suboxic early diagenetic conditions are indicated by corroded pyrite and newly formed glauconitic oncoids carbon and oxygen stable isotope data indicate cavity stabilization and cementation above the sulphate reducing zone oceanographic conditions during the time of cavity development are reconstructed from the carbon and oxygen isotope composition of cavity cements of bulk sediment and of associated ammonites 13 c values of up to 4 are related to the oceanic anoxic event of the early toarcian and oxygen isotope values from cements and ammonites define a shallow toarcian thermocline

dating and characterization of polymorphic transformation of aragonite to calcite in pleistocene bivalves from rhodes greece by combined shell microstructure stable isotope and electron spin resonance study absolute dating of diagenesis is a challenging yet significant problem reconstructing the depositional and diagenetic history of a late pleistocene coralligene type red algal reef and overlying maerl red algal sand from the plimiri coastal cliff se island of rhodes greece is enabled using a combination of microstructure stable isotope analyses and electron spin resonance of large bivalve shells partially altered by diagenesis these shells consisting of both original shell aragonite and polymorphic transformed rims now consisting of neomorphic spar provide the opportunity to date the primary age when the shells were precipitated as well as the age of their polymorphic transformation the contamination of samples presented a significant problem for interpreting ages based on data from electron spin resonance esr alone however sample contamination was evaluated with quantitative xrd providing the opportunity to apply corrections which increased reliability of the ages the carbon and oxygen stable isotope values and the neomorphic spar microtexture suggested that the polymorphic transformation at plimiri took place in a meteoric environment with a high and increasing fluid rock ratio the electron spin resonance dates combined with the vertical sedimentary evolution allowed to assign the formation of the red algal reef to the marine isotope stages 6 to 5e transgressive event ca 135 to 120 ka and to identify meteoric diagenetic phases during possibly the marine isotope stage 5d and the upper marine isotope stage 5c to lower 3 the use of electron spin resonance dating of neomorphic spar and age correction for sample contamination is presented herein for the first time and the results suggest that it might provide a useful tool to date late pleistocene diagenesis

late jurassic biogeochemical microenvironments associated with microbialite coated unionids bivalvia asturias n spain abstract the vega formation upper jurassic asturias spain contains two kimmeridgian outcrops the abeu and huerres sections with bivalves coated with carbonate microbialites oncoids the bivalve shells in the abeu section were replaced early by calcite while those of the huerres section were dissolved and the moulds filled by late diagenetic cements for the abeu shells there was a loss of sr and a gain of mg fe and mn during the replacement process but much of the original organic matter was retained part of the s present can be linked to this organic matter and part to the sulphate that replaced the carbonate groups in the calcite the thin microbialites inside the shells of some of the huerres bivalves have allowed us to estimate the influence of the growth rate on the incorporation of trace elements in calcite with an approximately six times lower growth rate the internal microbialite incorporates less mg and s and more fe and mn than the external one while maintaining the sr content the trace element content mg s sr fe and mn of the huerres and abeu microbialites are similar these coatings may therefore have been formed in the same type of freshwater but in different microenvironments given the substantial sulphate content of the microbialites from both sections this freshwater probably came from hot springs however the high fe mn ratio of the abeu microbialites suggests a reducing environment during their formation indeed 18o and 13c data confirm that the abeu and huerres microbialites were formed in different microenvironments the positive covariance for 18o and 13c of the abeu microbialites and replaced shells r2 0 83 and the wide range of variation for 18o 2 80 together suggest that the microbialite crusts grew in isolated ponds where evaporation was dominant in the huerres microbialites the positive covariance for 18o and 13c r2 0 59 and the narrow range of variation for 18o 0 74 indicate that the huerres section was formed in a more open environmental system where degassing predominated over evaporation

relic aragonite from ordovician silurian brachiopods implications for the evolution of calcification understanding the influence of aragonite calcite sea conditions on the evolution of biocalcification relies strongly on the correct interpretation of the original composition of calcareous taxa aragonite dissolves or inverts into calcite over geological time and its preservation is currently unknown to predate the pennsylvanian here we present direct evidence for the common occurrence of relic aragonite in ordovician and silurian trimerellid brachiopods thereby extending the known range of aragonite preservation by more than 130 million years together with associated hypercalcifying taxa of putatively original aragonite or high magnesium calcite composition and considerations of the temperature dependence of aragonite and calcite precipitation our results suggest that the evolution of aragonite biomineralization might have presented an adaptive advantage in shallow marine tropical waters of calcite seas a targeted search for paleozoic aragonite should both resolve the original composition of consistently recrystallized taxa and enable the reassessment of the aragonite calcite sea paradigm in a paleoenvironmental context

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today