evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

the influence of organic matter in organogenic dolomitization we hypothesize that the decomposition of organic debris derived from plants planktonic animals and algae by sulfate reducing bacteria and specifically by the enzymatic degradation of protein is vital to organogenic dolomitization the role of this degradation is to increase both alkalinity and ph of the pore waters providing the necessary solution and surface chemistries for dolomitization to occur ammonia derived by the enzymatic breakdown of proteins is the only simple base available to supply the ph conditions necessary for dolomitization protein is abundant in organic matter derived from a marine source because of the large enthalpies of hydration of mg super 2 and ca super 2 ions we propose that dolomitization proceeds with the transfer of neutral ion pairs caco 3 and mgco 3 to and from the dolomite surface within a replacement zone for this reason dolomitization will occur only when the concentration of the co 3 super 2 ion is high and in large excess to the hco 3 super ion in solution and at the carbonate crystal surface these conditions will be met when the ph and or ionic strength is high and the carbonate alkalinity is also high

mineral biosignatures in evaporites presence of rosickyite in an endoevaporitic microbial community from death valley california rosickyite is a rare form of sulfur sulfur monoclinic symmetry that is not thermodynamically predicted to be stable at earth s surface temperatures instead it reverts to the more common sulfur form orthorhombic symmetry here we show for the first time that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of death valley california we hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum caso4 2h2o to sulfide and reoxidation of the sulfide to elemental sulfur rosickyite within a stable oxygen sulfide gradient maintained by the organisms furthermore we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes

bacterially mediated formation of diagenetic aragonite and native sulfur in zechstein carbonates upper permian central germany abstract neoformed diagenetic aragonite associated with organic rich aggregations and native sulfur occur within cavities of the ca2 and ca3 zechstein carbonates in central germany the cavities were formerly filled with gypsum bacterial sulfate reduction favored the precipitation of the carbonate phase this can be attributed to an increase in alkalinity accompanied with sulfate reduction the high sr concentrations of the neoformed aragonites compared to the low concentrations in the carbonate host rocks point to sulfate dissolution as the cation supplying process for the precipitation of aragonite low 13c values 10 pdb of the aragonite indicate that some of its carbon is derived from organic matter that has been oxidized by bacterial sulfate reduction aragonite inclusions bear rhomb shaped crystals of calcite replacing former dolomite elevated mg ca ratios due to this dedolomitization may have promoted the precipitation of aragonite instead of calcite the aragonite precipitated in the near surface meteoric vadose zone in recent times aragonite crystals display a platy habit sem analyses show that two types of micro rods are associated with these plates the mineralized micro rods are interpreted to be fossilized bacteria aragonite inclusions most of which contain organic rich aggregations yield a distinctive biomarker pattern high concentrations of specific unsaturated fatty acids are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with the formation of the aragonite at one of the studied localities the aragonite is accompanied by native sulfur the formation of sulfur was mediated by h2s oxidizing bacteria this is corroborated by the presence of densely packed curved rods representing permineralized bacterial cells on and within the sulfur

geochemical modeling for predicting potential solid phases controlling the dissolved molybdenum in coal overburden powder river basin wy u s a abstract surface coal mining processes may increase the dissolved mo concentrations in overburden mine spoils and in nearby soils spoil and soil samples were subjected to solubility studies for determining potential solid phases controlling the dissolved mo additionally native soil samples were included in the study for comparison ion activity products iaps were calculated from the total elemental concentrations of aqueous extracts and compared with solubility products ksps of mo solid phases effects of dissolved organic carbon doc on metal ion complexation were also evaluated the ph of aqueous extracts ranged from 7 14 to 8 31 doc in aqueous extracts varied from 17 3 to 58 7 mg l molybdenum concentrations in aqueous extracts ranged between 11 5 and 35 6 g l for spoils 13 5 and 20 0 g l for soils near mine site and 14 5 and 29 6 g l for native soils initially iaps suggested varying degrees of supersaturation with respect to wulfenite pbmoo4 however when doc pb2 complexes were considered the iaps indicated a close approach to saturation with respect to pbmoo4 furthermore the iaps suggested a high degree of undersaturation with respect to powellite camoo4 and ferrimolybdite femoo4 results in this study suggest the dissolved mo concentrations in mine spoils and soils that are near coal mine may be controlled by pbmoo4 solid phase

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

pliocene lower pleistocene chronostratigraphy a re evaluation of mediterranean type sections the pliocene pleistocene stages defined for the mediterranean region are generally used as the standard chronostratigraphic units for the upper neogene and quaternary the recently developed calcareous plankton bio chronology based on results from ocean drilling program leg 107 and numerous outcrop sections provides the necessary time control for evaluating the ages and stratigraphic relationships for the various pliocene pleistocene stages and their associated stratotypes our results indicate that the currently accepted stratotypes do not provide a continuous stratigraphic representation for all of pliocene early pleistocene time on the basis of these findings we propose a revised chronostratigraphic scheme for this time period a tripartite subdivision is proposed for the pliocene series the zanclean stage is retained for the lower pliocene representing the interval from the miocene pliocene boundary ca 4 93 ma to ca 3 5 ma the middle pliocene is represented by the piacenzian stage and it incorporates strata deposited from ca 3 5 to 2 5 ma none of the currently used pliocene stages fully represents the upper pliocene ca 2 5 to 1 6 ma as a result late pliocene time lacks an adequately defined chronostratigraphic unit the selinuntian stage appears to be the most appropriate stage for the lower pleistocene ca 1 6 to 0 8 ma it can be divided into three stratigraphically continuous substages santernian emilian and sicilian the calabrian stage which is commonly used to designate the lowest part of the pleistocene should be abandoned because its stratotype does not include a record of the first 500 000 yr of pleistocene time

cross borehole slug test analysis in a fractured limestone aquifer summary this work proposes new semi analytical solutions for the interpretation of cross borehole slug tests in fractured media our model is an extension of a previous work by barker barker j a 1988 a generalized radial flow model for hydraulic tests in fractured rock water resources research 24 10 1796 1804 butler jr j j zhan x 2004 hydraulic tests in highly permeable aquifers water resources research 40 w12402 doi 10 1029 2003 wr002998 it includes inertial effects at both test and observation wells and a fractional flow dimension in the aquifer the model has five fitting parameters flow dimension n hydraulic conductivity k specific storage coefficient ss and effective lengths of test well le and of observation well leo the results of a sensitivity analysis show that the most sensitive parameter is the flow dimension n the model sensitivity to other parameters may be ranked as follows k le leo ss the sensitivity to aquifer storage remains one or two orders of magnitude lower than that to other parameters the model has been coupled to an automatic inversion algorithm for facilitating the interpretation of real field data this inversion algorithm is based on a gauss newton optimization procedure conditioned by re scaled sensitivities it has been used to interpret successfully cross borehole slug test data from the hydrogeological experimental site hes of poitiers france consisting of fractured and karstic limestones hes data provide flow dimension values ranging between 1 6 and 2 5 and hydraulic conductivity values ranging between 4 4 10 5 and 7 7 10 4 m s 1 these values are consistent with previous interpretations of single well slug tests the results of the sensitivity analysis are confirmed by calculations of relative errors on parameter estimates which show that accuracy on n and k is below 20 and that on ss is about one order of magnitude the k values interpreted from cross borehole slug tests are one order of magnitude higher than those previously interpreted from interference pumping tests these findings suggest that cross borehole slug tests focus on preferential flowpath networks made by fractures and karstic channels i e the head perturbation induced by a slug test propagates only through those flowpaths with the lowest hydraulic resistance as a result cross borehole slug tests are expected to identify the hydrodynamic properties of karstic channels and fracture flowpaths and may be considered as complementary to pumping tests which more likely provide bulk properties of the whole fracture karstic channel matrix system

characterization of the hydrogeological experimental site of poitiers france by stochastic well testing analysis summary we assess the applicability and performance of the stochastic well testing methodology of neuman et al neuman s p guadagnini a riva m 2004 type curve estimation of statistical heterogeneity water resources research 40 w04201 doi 10 1029 2003wr002405 neuman s p blattstein a riva m tartakovsky d m guadagnini a ptak t 2007 type curve interpretation of late time pumping test data in randomly heterogeneous aquifers water resources research 43 w10421 doi 10 1029 2007wr005871 to provide a geostatistical characterization of the transmissivity distribution of a heterogeneous aquifer we use cross hole pumping test information also referred to as interference testing collected during two extensive and detailed experimental campaigns performed in 2004 and 2005 in a limestone aquifer constituting the hydrogeological experimental site hes of poitiers france the methodology we use is based on stochastic type curves of mean drawdown and associated variance and is aimed at revisiting the results of routine pumping tests within a stochastic framework it allows estimating the geometric mean integral scale and variance of local log transmissivities on the basis of quasi steady state head data when a randomly heterogeneous confined aquifer is pumped at a constant rate the results are discussed by comparison against a previous deterministic analyses performed at the site using the same dataset and b information obtained by geostatistical analysis of existing geophysical data the estimated values of integral scales and variance of log transmissivity are viewed in light of multiscale theories which conceptualize the heterogeneity of porous and fractured systems as hierarchical random processes distributed across a multiplicity of scales the scale dependence observed for the estimated variogram parameters of log transmissivity at the hes site is consistent with the behavior of published worldwide data

a new method of data inversion for the identification of fractal characteristics and homogenization scale from hydraulic pumping tests in fractured aquifers hydraulic interference pumping tests are widely used for identifying mid to large scale aquifer hydraulic parameters fractured reservoirs often behave as fractal media for which classical interpretations may fail because of the decrease in permeability and fracture porosity or equivalently transmissivity and storage capacity with the pumping time and the extent over which the pumping stress has propagated a solution to hydraulic interference tests in fractal media is developed in dimensional variables which allows for a very rapid and unique fitting to data hydraulic parameters their scale exponents and the fractal dimension of the medium can be determined for each pressure draw down curve without any prior information or other calculation the method is applied to a large series of tests completed on the calcareous fractured aquifer of the experimental site in poitiers france even though data of pressure draw downs are various and show a fractal behavior permeability seems to rapidly homogenize for distances of 100 200 m on the other hand the fracture porosity shows a much progressive decrease with distance and homogenization is reached for scales beyond 300 500 m it is also shown that the classical cooper jacob interpretation of pressure draw downs may yield artifacts in the identification of permeability and fracture porosity orders of magnitude are comparable to that from fractal interpretation but their evolution with respect to time or distance scaling laws may strongly differ

textural and mineralogical biosignatures in an unusual microbialite from death valley california abstract for in situ astrobiological studies of mars or other planets we must employ strategies that will enable us to verify whether our approach and prototype instruments are actually capable of distinguishing life from non life this must be done against a background of rigorously conducted scientific characterization of the environment or sample types being considered for measurement by the instruments under development in this study we show how a combination of mineralogical and textural features can be considered a biosignature in an early mars analogue environment death valley california we propose that it is a combination of features in context of the geologic matrix which allows determination of biogenicity to be made polymineralic microbialites organosedimentary formations constructed by microorganisms from a spring pool at badwater within death valley national park are composed of alternating biogenic and abiogenic minerals in a distinct triplet sequence related to wet and dry seasons a microbial community occurring as a black biofilm produced paired layers of two different mineral types manganese oxyhydroxides and calcite these biogenic layers are separated from the next pair by a gypsum layer and appear to be laid down in the wet season with the gypsum a mineral positively identified on mars precipitating in the dry part of the year abiogenically i e not dependent on microbial metabolic activity for its deposition in addition textural features smaller grain size and less geometric morphology unique to the biogenic vs the abiogenic layers were consistently observed so that texture served as a biosignature in this environment

improving dual porosity medium approaches to account for karstic flow in a fractured limestone application to the automatic inversion of hydraulic interference tests hydrogeological experimental site hes poitiers france a reassessment improvement of a previous work by the authors is proposed on the inversion of hydraulic interference testing by means of dual porosity medium approaches the tests have been performed over a fractured limestone aquifer poitiers france showing a preferential and rapid flow path along open karstic drains all drawdown curves are merged together both in amplitude and time irrespective of their location relative to the single pumped well and classical dual porosity medium approaches are unable to mimic this behavior a pressure wave propagation by advection is added to the classical diffusion equations of dual porosity medium in order to stress the medium almost evenly whatever the distance from the pumped well the problem is set up to keep the continuous dual porosity medium framework with only two state variables i e the heads in fractures and matrix even though the wave velocity is not well conditioned because the ratio of the volume of drains to the total volume of the medium is unknown automatic inversion remains feasible the dual porosity medium without the drains can be homogeneous or fractal having in the latter case hydraulic parameters ruled by scaling power laws in both cases it is shown that adding an advection term allows to invert accurately all experimental drawdown curves statistical fluctuations of the sought hydraulic parameters are quite small and do not show any trend with respect to the distance thus modifying a dual porosity medium approach by adding an advection term allows for the homogenization of a reservoir that combines the general drainage of a fracture matrix system and a rapid drainage through karstic conduits

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

biological influences on modern sulfates textures and composition of gypsum deposits from guerrero negro baja california sur mexico abstract gypsum caso 4 2h 2 o deposits from a range of sedimentary environments at guerrero negro baja california sur mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent gypsum deposits were classified according to their sedimentary environment textures crystal habit brine composition and other geochemical factors the environments studied included subaqueous sediments in anchialine pools and in solar salterns as well as subsurface sediments of mudflats and saltpans gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles water column precipitates had euhedral prismatic habits and extensive penetrative twinning gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds gypsum precipitating within benthic biofilms and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits this evidence included irregular high relief surface textures accessory minerals s ca carbonate sr ca sulfate and mg hydroxide and distinctive crystal habits such as equant forms and crystals having distorted prism faces

a study of the lilla bredsjon tailings impoundment in mid sweden a comparison of observations with ratap model simulations abstract representative core samples have been collected from the lilla bredsjon tailings impoundment sweden the results of geochemical and mineralogical analyses are compared with the ratap model the study shows that the tailings can be divided into distinct zones the oxidation zone is characterized by oxidizing and acidic conditions associated with high concentrations of fe so 4 and other metal species in the pore solution acid neutralization by carbonates and hydraulic conditions in the tailings lead to abrupt changes in redox potential and sharp decreases in aqueous metal concentrations in the intermediate zone a hardpan layer 2 4 cm thick was identified the concomitant precipitation of gypsum jarosite goethite and ferrihydrite contribute to the formation of the hardpan the low rate of water percolation and the hardpan provide multiple controls on the formation of an oxidation barrier leading to low sulfide oxidation in the tailings this study shows that the ratap model is a useful tool to assist in predicting the impact of sulfidic tailings on the environment especially in tailings with clear zoning characteristics

hydrogeological characterization and modelling of weathered karst aquifers applicability to dewatering operations in limestone quarries limestone aquifers represent an important part of groundwater resources in the world they are usually considered as fractured karstic rock formations however a different type called weathered or ghost rock karst aquifer has recently been highlighted in this particular type of aquifer the karst is not expressed as open conduits but consists in zones made of a residual alterite resulting from the in situ partial dissolution of limestone these particular structures are crucial in some areas because they actually govern groundwater flow and transport in the aquifer they have however never been studied from a hydrogeological perspective in this study we present a methodology to characterize and model such aquifers results show that the ghost rock features are usually organized into networks and that their hydraulic properties are variable within each feature according to the limestone weathering intensity measurements in a specific aquifer in belgium are presented the modelling approach derived from the common equivalent porous medium approach enables to spatially distribute the variable hydraulic properties within the features from the hard fresh rock to the alterite in finite difference grids the method allows creating models including thousands of features and respecting correct connectivity relations the approach is considered at different scales with varying grid cells sizes results show that the induced error increases significantly when the size of the grid cells exceeds the width of the main features controlling flow the approach is applied on a case study consisting in dewatering operations in an exploited quarry located in a weathered karst aquifer presented results provide new insights regarding characterization and modelling of this kind of aquifers and highlight their importance in hydrological studies

relationships of modern gypsum nodules in sabkhas of loess to compositions of brines and sediments in northern egypt the host sediment is a desert loess forming a gypsiferous sonochak soil with a thin salt crust the nodules occur only in areas where the capillary fringe intersects the ground surface usually where the surface is within 1 5 m of the water table nodules a few centimeters beneath the surface are white very fine grained gypsum nodules at lower levels are of more coarsely crystalline gypsum phreatic groundwater brines although saturated for calcium sulfate are only moderately hypersaline about 60 per mil new evidence suggests that they are mainly of marine origin surface evaporation causes capillary brines above the water table to move upwards through the loess and to increase in salinity a marked increase in the m mg super 2 m ca super 2 ratio of the interstitial brines at the zone of fine grained nodules demonstrates active precipitation there of calcium sulfate it is because of the existence of a very high salinity zone 300 per mil with halite at the top of the profile that gypsum precipitation takes place beneath the surface so as to form nodules rather than a surface crust na super and cl super ions in addition to ca super 2 and so 2 super 2 ions seem to be necessary for the development of gypsum nodules by the per ascensum mechanism modified journal abstract

christmas island lagoonal lakes models for the deposition of carbonate evaporite organic laminated sediments the atoll of christmas island now known as kiritimati in the kiribati republic central pacific lies at about 28n in the intertropical convergence zone much of the surface area of the atoll ca 360 km 2 is occupied by numerous lakes in which carbonate evaporite calcium sulfate halite and organic layers are deposited observations suggest that deposition of these different laminae is controlled by climatic and biologic factors it is thought that periodic climatic variations such as el nino southern oscillations enso events which bring heavy rainfall to the atoll result in the succession of the precipitation of carbonate minerals during periods after dilution of hypersaline waters by heavy rains followed by evaporitic minerals carbonate calcium sulfate halite when salinity increases through evaporation thick up to 5 cm microbial essentially cyanobacterial mats develop continuously on the lake bottom surfaces providing the sediment with an important total organic carbon 2 5 organic contribution in the form of an internal geometrically structured network in which the authigenic minerals precipitate the high bioproductivity of these microbial populations is reflected in low d 13 c values of sedimentary organic carbon 214 to 217 interpreted as being the result of high atmospheric co2 demand geochim cosmochim acta 56 1992 335 the well laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events storms heavy rainfall these lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment the high ratio of deposited carbonate vs sulfate 1 chloride when compared to low ratio in evaporitic salinas results from both a lack of limitation of calcium magnesium and carbonate ions in a carbonate reef environment and active processes of high mg calcite precipitation organomineralization q 2001 elsevier science b v all rights reserved

predictive modelling of hydraulic head responses to dipole flow experiments in a fractured karstified limestone aquifer insights from a comparison of five modelling approaches to real field experiments summary five modelling approaches were tested against a well studied limestone aquifer constituting the hydrogeological experimental site hes of poitiers france the modelling exercise consisted of predicting the drawdown responses of a series of observation wells for two dipole pumping injection flow experiments involving two distinct well pairs at the hes the differences between model predictions appear to be mainly related to the hydraulic datasets used for model parameterisation and to a much lesser extent to the conceptual modelling approach equivalent porous medium vs discrete fracture networks the model dimensionality two dimensional vs three dimensional and or the parameterisation approach forward vs inverse despite the abundance and diversity of calibration parameterisation data all of the models failed to predict the drawdowns with a reasonable degree of accuracy only the order of magnitude of the drawdowns was correctly predicted by three of the five models whereas all models failed to predict the drawdown behaviour at both intermediate and late times the primary source of error is attributed to a lack of information in the single well pumping test data which did not capture the multi permeability structure of the aquifer as revealed a posteriori by the responses to dipole flow experiments this highlights the need to develop novel or improved approaches to characterise the hydraulic properties of limestone aquifers

mediation of endoevaporitic microbial communities in early replacement of gypsum by dolomite a case study from miocene lake deposits of the madrid basin spain abstract based on petrographic mineralogical isotope and facies assemblage analysis a microbial origin is established for the formation of dolomite associated with gypsum in miocene evaporite lake deposits of the madrid basin central spain in these deposits dolomite is present as both intercalated carbonate beds locally showing domal stromatolite structures between packages of selenite christmas tree like gypsum and patches replacing macrocrystalline gypsum texture of the dolomite is characterized by crystal aggregates showing a variety of crystal sizes and morphologies e g platelets rhombs micro rods and rings whilst larger crystals are commonly spherical and or wheat grain shaped organic remains in the form of filaments shrubs micro fibrils and strands are also common and contain significant amounts of carbon these textural features are also recognized in dolomite replacing gypsum where fe oxide and sulfide as well as celestite are ubiquitous mixed with the dolomite groundmass the dolomite whether primary or replacing gypsum is poorly ordered and slightly ca rich thus non stoichiometric stable isotope compositions are characterized by negative values for both oxygen and carbon dolomite beds featuring domal stromatolites have 18o values ranging from 2 99 and 3 79 and 13c values ranging from 4 67 and 7 35 whilst 13c values determined in the dolomite replacive of gypsum shows a small range of variation between 5 70 and 6 96 by contrast 18o values of replacive dolomite oscillate in a wider range from 3 04 to 7 99 formation of the dolomite was associated mainly with microbial mats having taken place in relatively diluted lake water further evaporative concentration resulted in precipitation of gypsum crystals sealing the mats and creating endoevaporitic microenvironments in which endolithic cyanobaterial activity produced extensive boring and corrosion of the gypsum crystals hiatuses in gypsum growth caused an intensification of the corrosion process and favored the precipitation of dolomite mediated by microbes resulting in pervasive replacement of the sulfate

trace element and isotope geochemistry of zoned calcite cements lake valley formation mississippian new mexico insights from water rock interaction modelling abstract regionally extensive pre pennsylvanian meteoric zoned calcite cements from the mississippian age lake valley formation new mexico comprise three major zones zones 1 2 and 3 each with a distinct isotope and trace element geochemistry zone 1 has the highest mg 18o and 13c contents zone 2 intermediate values and greatest mn and fe and zone 3 has the lowest trace element 18o and 13c values covariation of c and o isotopes for the three zones define an inverted l shaped trend and covariations of mg mn and mg fe define l shaped trends all suggestive of rock evolution curves resulting from water rock interaction these covariant trends were the impetus for quantitative mass balance modelling of water rock interaction iterative mass balance modelling of covariations of stable isotopes c and o and trace elements mg mn fe of these zoned cements implies that water rock interaction progressively decreased water rock ratios increased during precipitation cementing waters for the oldest cement zone zone 1 resulted from water rock ratios in the range of tens to a few hundred whereas the youngest cement zone 3 resulted from water rock ratios in the 100 s to 1000 s the modelling also implies that differences in geochemistry of the cement zones cannot be explained solely by changes in water rock ratios but require greater 18o and greater ca and mg concentrations in input waters that evolved to zone 1 precipitational waters than those for zones 2 and 3 zones 2 and 3 can reasonably be modelled using input waters with the same isotopic compositions the same ca concentration and for some zone 2 cement the same mg concentrations the minimum mg ca ratios in input waters is constrained by the lower mg plateau in mg mn space and is about the same for all three zones about 1 25 the maximum mg ca ratios about 1 20 occurred in zone 2 input waters mn fe covariations also show an l shaped trend a trend whch fits neither a simple water rock interaction model nor a simple eh model a model is proposed in which eh and limited mn sources determine the amount of fe and mn available in solution to partake in water rock interaction water rock interaction modelling provides one useful approach to quantitatively interpreting isotopic and trace element data from diagenetic carbonates to gain a clearer understanding of evolution of diagenetic fluids

inversion of a dual continuum approach to flow in a karstified limestone insight into aquifer heterogeneity revealed by well test interferences summary two large sets of interference testing data from a karstic aquifer hydrogeological experimental site hes poitiers france are inverted using a dual continuum approach to groundwater flow the parameterization technique used to infer spatially distributed hydrodynamic parameters is free of any prior estimation regarding the spatial structure of the parameter fields it merely seeks the minimal number of local values to obtain the best fit between flow simulations and data a previous work inverting without any prior geostatistical assumption the same set of data based on a single flowing continuum produced nicely structured spatially correlated parameter fields the dual continuum approach renders fields as patchworks of subareas of quite uniform values the statistical distribution of the hydrodynamic parameters is generally skewed toward high values especially when interference data bear traces of rapid channeled flow in karstic conduits it is now well established that continuous approaches to flow are well suited to simulate diffusive flow in fractured karstified rocks although single continuums may result in artificially high contrasts of hydraulic diffusion values between flowing and non flowing areas in the case of a dual continuum the model has in its physics the capability to clearly separate conduction properties attached to fractures and channels from capacitive properties of the host matrix we show that high contrasts of hydraulic diffusion disappear resulting in a mildly heterogeneous medium in which high values of hydraulic diffusion are only located in areas where channeled flow occurs

abu dhabi sabkha persian gulf revisited application of strontium isotopes to test an early dolomitization model a study of brines and diagenetically altered minerals from coastal sabkha of abu dhabi persian gulf was undertaken to test the flood recharge evaporative pumping model for early dolomitization using strontium isotopes as a tracer the sup 87 sr sup 86 sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model the sup 87 sr sup 86 sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters the sup 87 sr sup 86 sr ratio of gypsum from the dolomitization area 1 4 to 4 4 km from high tide mark and seaward reflects precipitation from marine brines but more landward 6 to 9 km gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence dolomite yields gypsum yields anhydrite the dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes

interference pumping tests in a fractured limestone poitiers france inversion of data by means of dual medium approaches hydraulic interference pumping tests remain a valuable method for obtaining information on underground flows at a scale compatible with the operation of a reservoir combining a dual medium model with the inversion of interference testing data in fractured rocks is very recent probably because the model is ruled by very contrasted sensitivities to its main parameters it is shown that an accurate and stable inversion requires a specific procedure based on re scaled sensitivities to parameters both homogeneous and fractal dual media have been inverted on two sets of data obtained from the fractured limestone of the experimental site in poitiers france the first data set can be homogenized with both homogeneous and fractal approaches yielding almost constant hydraulic parameters or only weak statistical variations beyond experienced distances of 200 m the second data set collected in a site perturbed by forced flows from the first campaign shows preferential flow paths with a piston like propagation of the pressure depletion only the fractal dual medium is able to partly homogenize the hydrodynamic behavior surprisingly the hydraulic conductivity is constant but the dimensioning constants of the capacity parameters continue to decrease with the size of the system thus the intrinsic decrease of parameters with the size of the system in the fractal approach is not sufficient to mimic the scale effects recorded by interference tests superimposition of piston like effects onto dual media shall be further investigated to better account for the influence of scarce flow paths with very high velocities

mineralogical footprints of microbial life earth s geosphere is intimately tied to its biosphere a major link between the two lies in the microbial realm microorganisms grow in and upon rocks and minerals often relying on their substratum for critical compounds needed in order to produce cellular energy the presence of a metabolizing cell on a mineral substrate has a significant effect on the mineral texture and on the geochemistry of the surrounding microenvironment in nature microorganisms exist in microbial commu nities as mats or biofilms growing upon a solid substrate as such they cover a vast surface area both within and below the surface of earth s land and sea the following review will provide a glimpse into the latest findings in the field of geomicrobiology and is intended to convey a sense of the profound influence microorganisms can have upon the geological environment they inhabit

heating and freezing experiments on aqueous fluid inclusions in anhydrite recognition and effects of stretching and the low temperature formation of gypsum aqueous fluid inclusions in anhydrite that are frozen during microthermometry commonly develop new daughter crystals heating these inclusions induces the new daughter crystals to decompose repeated cycles of crystal growth and decomposition may result in morphological changes of the fluid inclusions this phenomenon of new mineral growth presumed to represent the low temperature hydrate gypsum has been recognized for over 20 years nevertheless its occurrence and its significance to the determination of inclusion salinities are not widely appreciated similarly the potential of inclusions in anhydrite to stretch when overheated is commonly assumed yet tests demonstrating the phenomenon have not been reported changes in the ice melting temperature of aqueous inclusions in anhydrite before and after gypsum formation were documented showing that hydration of the host anhydrite may significantly lower the ice melting temperature and increase the salinity of the residual inclusion fluid heating inclusions to induce gypsum to decompose results in the release of bound water restoring the inclusions to their original salinity the homogenization temperatures of inclusions that have undergone these phenomena are generally unchanged indicating that the inclusions specific volumes are unchanged however heating aqueous inclusions in anhydrite more than ten degrees or so above their homogenization temperature commonly results in stretching therefore those who study aqueous inclusions in anhydrite must avoid the determination of ice melting temperatures if new gypsum daughter crystals are present and must determine homogenization temperatures sequentially with rising temperature to avoid overheating and stretching

endolithic cyanobacteria in soil gypsum occurrences in atacama chile mojave united states and al jafr basin jordan deserts 1 soil sulfates are present in arid and hyperarid environments on earth and have been found to be abundant in soils on mars examination of soil gypsum from the atacama desert chile the mojave desert united states and al jafr basin jordan revealed endolithic cyanobacteria communities just below the surface of soil gypsum samples optical and scanning electron microscope observations of the colonized layers indicated that the unicellular chroococcidiopsis is the dominant cyanobacterium in all studied communities 16s rrna gene analysis revealed that in addition to chroococcidiopsis a few other cyanobacteria are present heterotrophic bacteria are also abundant in the colonized zones of the fine grained gypsum from the atacama and mojave desert but insignificant in the fibrous gypsum from the jordan desert endolithic life forms similar to these described here may exist or have existed on mars and should be targeted by the mars science laboratory and future in situ missions

near infrared spectroscopy of lacustrine sediments in the great salt lake desert an analog study for martian paleolake basins the identification and characterization of aqueous minerals within ancient lacustrine environments on mars are a high priority for determining the past habitability of the red planet terrestrial analog studies are useful both for understanding the mineralogy of lacustrine sediments how the mineralogy varies with location in a lacustrine environment and for validating the use of certain techniques such as visible near infrared vnir spectroscopy in this study sediments from the pilot valley paleolake basin of the great salt lake desert were characterized using vnir as an analog for martian paleolake basins the spectra and subsequent interpretations were then compared to mineralogical characterization by ground truth methods including x ray diffraction automated scanning electron microscopy and several geochemical analysis techniques in general there is good agreement between vnir and ground truth methods on the major classes of minerals present in the lake sediments and vnir spectra can also easily discriminate between clay dominated and salt dominated lacustrine terrains within the paleolake basin however detection of more detailed mineralogy is difficult with vnir spectra alone as some minerals can dominate the spectra even at very low abundances at this site the vnir spectra are dominated by absorption bands that are most consistent with gypsum and smectites though the ground truth methods reveal more diverse mineral assemblages that include a variety of sulfates primary and secondary phyllosilicates carbonates and chlorides this study provides insight into the limitations regarding the use of vnir in characterizing complex mineral assemblages inherent in lacustrine settings

trace elements and their behaviour during the combustion of marine oil shale from changliang mountain northern tibet china the shengli river changshe mountain oil shale zone represents a potentially large marine oil shale resource in china with the aim of better understanding the geochemistry of trace elements in marine oil shale and its combustion residues 40 raw samples 27 oil shale combustion residues and 29 selected minerals from changliang mountain oil shale are studied for geochemical analyses the contents of se cd mo as cs pb sr and u in the oil shale samples are enriched from 1 47 to 33 91 times as compared with the clarke values whereas the concentrations of other elements are slightly higher lower than the respective worldwide means the most enriched elements in oil shale combustion residues are se cd mo as and u with enrichment values from 4 78 to 50 92 trace elements with high volatile behaviour such as as co ni sc sn and v occur predominantly in organic matter and or sulphides other non volatile or slightly volatile trace elements e g be bi cs cu ga hf li nb rb ta th w zr and rees may occur mainly in original and relatively refractory minerals in raw oil shale the potentially hazardous trace elements in changliang mountain oil shale include as cd mo and se arsenic and se are controlled mainly by fe bearing minerals probably pyrite in changliang mountain oil shale cadmium is present mainly in dolomite while mo occurs mainly in organic matter

conceptualization of flow and transport in a limestone aquifer by multiple dedicated hydraulic and tracer tests abstract limestone aquifers are of great interest as a drinking water resource in many countries they often have a complex crushed and fractured geology which makes the analysis and description of flow and transport processes in such aquifers a challenging task in this study the solute transport behavior including fracture matrix interaction in hydrogeological units of a limestone aquifer in eastern denmark was characterized by designing conducting and interpreting six depth specific tracer tests involving natural and forced gradient conditions with multiple tracers representing different diffusion properties to determine flow parameters the tracer tests were complemented by a comprehensive set of depth specific borehole and hydraulic tests based on the tests a new and stronger conceptual understanding was developed for the different aquifer units the investigated limestone aquifer is composed of a glacially crushed unit and two fractured units with calcarenitic and bryozoan limestone of similar hydraulic properties hydraulic tests revealed that the crushed unit has a lower hydraulic conductivity than the fractured limestone units likely due to the crushed conditions with small limestone clusters and small aperture fractures potentially filled with fine material in the fractured limestone units a distinct preferential flow and primary transport along major horizontal fractures was inferred from the tracer tests under forced gradient conditions the dominant horizontal fractures were identified on impeller flow logs and appear connected between wells having an extent of up to several hundred meters connectivity between the aquifer units was investigated with a long term pumping test and tracer tests revealing restricted vertical flow and transport a very pronounced hydraulic conductivity contrast between major fractures and matrix could also be inferred from the borehole and hydraulic tests which is consistent with the findings from the tracer tests the difference in the matrix diffusion behavior of the simultaneously injected tracers and a long tailing in the breakthrough curves revealed that matrix diffusion has a strong influence on the solute transport in the fractured limestone

three dimensional interference test interpretation in a fractured aquifer using the pilot point inverse method a geostatistical conditional simulation routine that employs both parametric and nonparametric geostatistics is coupled to the pilot point inverse method to simulate the spatial distribution of conductivities in a dolomitic aquifer which is fractured in some areas and unfractured in others an inversion is then conducted to obtain the conductivities within the fractured and nonfractured parts of the aquifer to match interference data from a series of three dimensional pumping tests for an ensemble of 100 conditional simulations the calibrated and predicted drawdowns match the observed drawdowns well a comparison to the transmissivity interpreted from a single well pumping test compared very well to the geometric mean transmissivity calculated from the calibrated realizations in this study the results from this study indicate that conditioning the conductivity fields to the geologic facies data in this case fractured unfractured categorical data as well as to ample transient hydraulic data can lead to robust groundwater flow models which can adequately predict the response to other hydraulic interference tests

inversion of a set of well test interferences in a fractured limestone aquifer by using an automatic downscaling parameterization technique summary this work inverts interference hydraulic tests from a karstic aquifer modeled as a single continuum with heterogeneous hydraulic properties hydraulic conductivity k and specific storage capacity ss an automatic technique of parameterization is proposed to solve the inverse problem sought parameters are located at the nodes of a parameter grid made of triangular elements and independently superimposed onto the calculation grid of the flow problem the parameter grid can be refined within the process of inversion so that the solution is improved locally because increasing the number of parameters better depicts spatial heterogeneity applied to hydraulic tests carried out at the hydrogeological experimental site hes of poitiers france this adaptative multi scale triangulation amt method of parameterization reveals that hydraulic conductivity k and hydraulic diffusion k ss are structured in space as correlated stochastic continuums this structure is not imposed a priori but is found as the mere consequence of inverting drawdown data the sought stochastic continuums are discussed and compared to previous results obtained by processing the same data with various techniques even though interference data show evidences of local karstic features the hydraulic bulk behavior of the aquifer is that of a diffusive continuous medium this reinforces the idea that at mid to large scales flow in fractured rocks can also be handled with relevance by means of a continuous heterogeneous approach this result would extend to fractured aquifers with local draining in open conduits as is the case in this study

the depositional and diagenetic evolution of the coastal ridges of northwestern egypt since the lower pliocene successive generations of coastal ridges made up of mixed oolitic and biogenic grainstones formed along the mediterranean coastline of egypt west of alexandria and have undergone progressive stages of diagenesis the ridges are complex structures resulting from several phases of deposition in which shallow marine components are partially and progressively reworked as subaerial and aeolian deposits diagenetic alteration resulted from increasing freshening of the pore waters in the ridges diagenesis began with marine phreatic cementation followed by freshwater phreatic cements aggrading recrystallization of microcrystalline calcite to microspar and minor dolomitization non marine hypersaline diagenesis soil profiles developed on the upper surfaces of the oldest dunes

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

mineralogy of a mudstone at yellowknife bay gale crater mars sedimentary rocks at yellowknife bay gale crater on mars include mudstone sampled by the curiosity rover the samples john klein and cumberland contain detrital basaltic minerals calcium sulfates iron oxide or hydroxides iron sulfides amorphous material and trioctahedral smectites the john klein smectite has basal spacing of similar to 10 angstroms indicating little interlayer hydration the cumberland smectite has basal spacing at both similar to 13 2 and similar to 10 angstroms the larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating h2o retention basaltic minerals in the mudstone are similar to those in nearby eolian deposits however the mudstone has far less fe forsterite possibly lost with formation of smectite plus magnetite late noachian early hesperian or younger age indicates that clay mineral formation on mars extended beyond noachian time

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

minerals and potentially hazardous trace elements in marine oil shale new insights from the shengli river north surface mine northern tibet china the shengli river changshe mountain oil shale zone including the changliang mountain shengli river oil shale the shengli river north oil shale and the changshe mountain oil shale represents potentially the largest marine oil shale resource in china new geochemical data from the shengli river north oil shale are presented to evaluate potentially hazardous trace elements in oil shale and to discuss possible factors controlling the geochemical anomalies oil shale samples from the shengli river north area are characterized by high ash yield 59 25 66 27 and toc content 4 77 9 35 with low total sulfur content 0 1 1 56 and intermediate shale oil content 3 60 16 30 minerals in the shengli river north oil shale include calcite clay minerals quartz dolomite and fe bearing minerals e g pyrite limonite and hematite and trace amounts of zeolite apatite and barite potentially hazardous trace elements in oil shale samples include as se mo f ni cd and hg arsenic is present probably in both organic and inorganic matter selenium is controlled mainly by fe bearing probably pyrite while f and cd occur mainly as apatite the organically bound mo and ni are dominant and hg probably occurs as a substitution in the structure of pyrite apatite and calcite the elevated concentrations of f and cd in the shengli river north oil shale may be related to sediment source water chemistry may have played an important role for ni and mo enrichments while the enrichment of se in the oil shale samples is possibly related to hydrothermal fluids other elements including hg and as are probably related to several factors

biogenic carbonates are they a criterion for underlying hydrocarbon accumulations an example from the gulf of suez region egypt the gulf of suez petroliferous basin is characterized by the occurrence of a middle upper miocene evaporite cap field and subsurface investigations in the southwest gulf of suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur the first type of carbonate is biogenic syngenetic in origin and was formed in low temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate reducing bacteria the resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum calcite sulfur or as sulfur ooids sulfur globules and stromatolitic sulfur that are enclosed in gypsum or calcite the biogenic syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings the second type of carbonate is a biogenic epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying miocene evaporites the carbonate is vuggy is included within or caps the evaporites and usually shows hydrocarbon stains or emits a hydrocarbon smell the sulfur is usually yellow but may be brown to black due to bitumen staining carbonates replace gypsum or anhydrite and occur as aragonite calcite or dolomite the d13c mean values for carbonate minerals range from 7 8 to 27 2 pdb being relatively heavier than the d13c values of the nearby crude oil 28 1 to 29 3 pdb carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions where the lighter methane is stable the occurrence of biogenic epigenetic carbonates and sulfur in the evaporite deposits of the gulf of suez petroliferous basin is similar to that recorded in other basins such as the gulf coast basins of texas louisiana and southern mexico the fergana and amudarya depression of central asia and the mesopotamian depression in iraq the biogenic epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon bearing reservoir at depth

magnesite formation by microbial activity evidence from a miocene hypersaline lake abstract this paper provides an ancient analogue for biologically mediated magnesite in lacustrine hypersaline environments thin beds of massive to crudely laminated magnesitic marls occur interbedded with mudstone and evaporite facies deposited in a saline lake mudflat sedimentary system during the lower miocene in the madrid basin central spain exposure of this succession in a recently excavated tunnel and in collected cores offered a good opportunity to study magnesite precipitates that have been preserved in unaltered conditions by primary evaporitic minerals specifically magnesite arranged as host matrix between and enclosed by displacive halite and glauberite crystals occurs closely associated with abundant microfossils embedded in sheets of organic matter interpreted as the remains of extracellular polymeric substances eps even many magnesite clots are seen to have resulted from the agglutination of fossilized bacterial bodies forming a biofilm the close and pervasive association of microorganisms and magnesite is used as evidence that microbes played a fundamental role in the precipitation of this mineral additional traces of microorganisms and microbial activity observed in magnesite beds include the isotopic ratios in the carbonates with the 13c pdb value averaging 6 2 the presence of carbonaceous film like structures and the association with patchly distributed pyrite and celestite minerals on the basis of the combined sedimentological mineralogical chemical and morphological signatures of the magnesitic deposits it is postulated that carbonate crystals precipitated in microbial mats magnesite crystals together with celestite local barite and or pyrite have been also observed to replace silicate and sulphate minerals it is suggested that magnesite precipitation was biochemically coupled with the early dissolution of the associated sedimentary minerals microbes may have used the associated minerals as source of energy and or of essential elements