texture of microbial sediments revealed by cryo scanning electron microscopy abstract textures of modern lacustrine stromatolites on kiritimati line islands central pacific ocean and of buried layers in the stromatolitic carbonate sediments from french polynesian atoll lakes kopara have been studied using cryo scanning electron microscopy sem equipped with a freeze drying sample preparation device this study confirms that microscopic three dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria and from other extracellular polymer secretions are common components of microbial sediments of which they may form the framework in addition to this role in sediment cohesion and formation of microstructure the organic framework appears to be involved in carbonate precipitation with n the stromatolites through chemical nucleating steric and hydrodynamical controls the role of the dead organic constituents of the stromatolites is not only confirmed in micrometer size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments and to the formation of peloids that evolve into spherulites bacterial including nannobacterial carbonate bodies and carbonate impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied all these carbonate precipitation processes may cooperate in lithification of a given sediment

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

abiotically formed primary dolomite in the mid eocene lacustrine succession at gebel el goza el hamra ne egypt an approach to the role of smectitic clays abstract this study discusses the role of smectitic clays in the formation of an abiotic physio chemical primary dolomite within an evaporative alkaline saline marginal lake system in the absence of carbonate precursor and microbes the present work has been achieved in terms of textural mineralogical and geochemical characteristics of dolostones in the mid eocene bartonian lacustrine succession cropping out at gebel el goza el hamra shabrawet area ne egypt this lacustrine succession is 15 16 m thick and made up of alternating horizontal beds of dolostone marlstone and mudrock that show some pedogenic and subaerial exposure features the dolostones are composed mainly of dolomite 60 90 smectite 20 30 and quartz grains 5 10 the dolomite comprises fine crystalline rhombs to micro spherical crystals with no obvious relics of microbial activity and or carbonate precursor it is ordered nearly stoichiometric with 46 50 mole of mgco 3 and has 18 o and 13 c values ranging from 0 44 to 2 96 vpdb and 0 93 to 8 95 vpdb respectively the smectite occurs as thin mats that are commonly intergrown and associated with dolomite mineralogical textural and stable isotopic results of the dolomite indicated that the dolomite was formed as an abiotic primary precipitate in alkaline saline lacustrine systems in this respect the gel like highly viscous smectitic medium plus progressive co 2 degassing elevated evaporation low sedimentation rate low sulphates level and alkaline soil solution lowered the kinetic barriers of dolomite precipitation from solution and promoted the incorporation of mg 2 in the structure of dolomite consequently the presence of smectitic clays in evaporative saline lakes is significant for dolomite formation because they can generate a gel like highly viscous medium and provide mg 2 that can facilitate the physcio chemical precipitation of primary dolomite from solution at ambient temperatures however more work is needed to better characterize the role of clays during dolomite formation in alkaline lacustrine environments at ambient temperatures

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

dolomite formation and biogeochemical cycles in the phanerozoic summary based on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation the abundance and distribution of dolomite throughout the phanerozoic remains an enigma sometimes referred to as the dolomite problem comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it the recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective a biogeochemical approach to the dolomite problem reveals a plausible connection between phanerozoic geochemical cycles and dolomite formation in particular periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans lowered oxygen levels would have fostered a more active community of anaerobic microbes including sulphate reducing bacteria which in turn could have led to more extensive dolomitization of marine carbonates

microbial dolomite in fresh water carbonate deposits this article reports evidence for biologically mediated precipitation of dolomite in a fresh water spring in the south east of spain build ups in the spring consisted of calcite minor dolomite barite and gypsum crystals formed by the mineralisation of microbial biofilms and bryophytes a detailed microscopic study coupled with geochemical analysis of the dolomite precipitating biofilms showed that dolomite crystals occurred in association with living coccoid micro organisms providing strong evidence that micro organisms play a fundamental role in the precipitation of this mineral many of the calcite crystals observed were associated with extracellular polymeric substances moreover microbial involvement in the precipitation of calcite was further supported by the presence of living cyanobacteria within the calcite crystals two different types of build up were found in the spring soft spongy moss tufa and laminated crusts two types of crusts were identified as follows simple pink crusts and laminated crusts the latter forming in both sub aquatic and sub aerial environments sub aquatic accumulation of simple detached pink crust fragments occurred sporadically both tufa and the carbonate crusts hosted cyanobacterial dominated biofilms with different bacteria and diatoms and their extracellular polymeric substances the 13c values of calcite showed some biogenic involvement in the origin of this mineral the 18o values were similar in the tufa and carbonate crusts and corresponded to current climatological characteristics the occurrence of different groups of minerals carbonates and sulphates in close spatial association with micro organisms suggests the presence of different macro environmental and micro environmental conditions that facilitate mineral precipitation within biofilms these findings extend the known range of aerobic microbial dolomites to include springs and show that the precipitation of both dolomite and calcite carbonates occurs while organisms are alive

early middle triassic stromatolites from the luoping area yunnan province southwest china geobiologic features and environmental implications abstract an early middle triassic stromatolite deposit is documented from the guanling formation of the luoping area yunnan sw china the luoping stromatolite shows five types of constructional microbial forms in various magnifications 1 typical stratified columnar structures up to 20 cm high with crinkled laminae dark coloured laminae 1 mm thick are composed of upright filamentous tubes averagely 29 4 m in diameter showing a vertical growth fabric 2 laminoid fenestrae 0 5 1 mm wide and 3 prostrate filaments which are reflected by strong fluorescence in sharp contrast to dolomite cement in fluorescent images 4 rod like aggregates 4 6 18 m in diameter composed of minute dolomite rhombs are very common in stromatolitic laminae they resemble present day cyanobacterial trichome and thus may represent putative fossilized cyanobacteria 5 moreover small pits coccoid like spheroids calcified biofilms and fibrous structures are also common in stromatolitic laminae the last two may represent calcified extracellular polymeric substances eps that contributed to the development and lithification of stromatolites authigenic quartz grains are common and may also have involved biological processes in stromatolite formation of these microbial functional groups driving accretion and lithification processes of stromatolite documented in literature both lithified cyanobacteria oxygenic phototrophs and possible sulfate reducing bacteria srb which induced microbial formation of dolomite and contributed to the accretion of the luoping stromatolite are suggestive of biogenic origin the luoping stromatolite differs from the permian triassic boundary microbialites ptbms in having abundant filamentous structures and growing in an oxic marine environment both sedimentary facies analysis and abundant fossilized cyanobacteria may indicate proliferation of oxygenic phototrophs in a normal oxic habitat during the middle anisian early middle triassic a period when hospitable environments coupled with biotic diversification prevailed in south china and set an agenda for the full recovery of marine ecosystems in middle late anisian however the post extinction stromatolites and other anachronistic facies are not necessarily indicative of anoxic or oxic conditions and their environmental settings are much more complex than previously thought

sedimentology and stable isotopes from a lacustrine to palustrine limestone deposited in an arid setting climatic and tectonic factors miocene pliocene opache formation atacama desert chile abstract field relations petrography gis and geochemistry of a late miocene pliocene limestone the opache formation calama basin atacama desert chile shows that depositional facies diagenetic histories and isotopes differ from eastern to western parts of the basin we recognize two facies lacustrine characterized by ostracod diatom mudstone and wackestone and palustrine characterized by both shallow water and pedogenic features gis analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west with the calama constriction restricting flow between east and west petrography and geochemical results confirm that the calama constriction influenced depositional parameters ponding groundwater to the east forming a shallow lake and creating palustrine conditions to the west our analyses indicate that lake waters were less enriched in 13c or 18o than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history derived from groundwater rich in dissolved minerals this seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the pacific ocean and was less evolved whereas eastern limestones precipitated from water which had become depleted in 18o during transport from the east today moisture from the pacific very rarely if ever reaches inland as far as calama indicating that during the late miocene mid pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal pacific current allowed moisture from the pacific to reach inland this changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the andes allowing it to reach farther westward into the atacama region during the late miocene pliocene together these moisture sources were sufficient to produce carbonate rich palustrine and lacustrine environments in the calama basin

climate and biomes in the west mediterranean area during the pliocene a new pollen derived method of climatic quantification based on the mutual climatic range of plant taxa has been applied to 17 pliocene pollen sequences in the west mediterranean area the latitudinal gradient observed in the pollen data was confirmed by the climatic reconstructions there is a gradient from north to south both for temperatures and precipitation at the beginning of the pliocene 5 32 5 ma the climate in the north mediterranean area was on average warmer and more humid than today respectively 1 4oc and 400 700 mm in the south mediterranean region the climate was both warmer and drier than today respectively equal to or 5oc higher and drier or equal humidity the pollen based climate estimates were then used to reconstruct biomes for the region the results show the occurrence of three biomes the broad leaved evergreen warm mixed forest the xerophytic woods scrub biome and the warm grass shrub biome these biomes are still represented today in the mediterranean region despite different thermic and water conditions

mid eocene alluvial lacustrine succession at gebel el goza el hamra shabrawet area ne eastern desert egypt facies analysis sequence stratigraphy and paleoclimatic implications abstract this study aims to provide the depositional facies sequence stratigraphic and paleoclimatic characteristics of the mid eocene bartonian continental succession exposed at gebel el goza el hamra shabrawet area ne eastern desert egypt the studied succession consists of siliciclastic rocks followed upward by carbonate rocks detailed field observation and petrographic investigation indicate accumulation in floodplain dominated alluvial and shallow lacustrine systems the floodplain dominated alluvial facies 45 m thick is composed mainly of carbonate nodules bearing mottled mudrock with subordinate sandstone and conglomerate beds the conglomerate and pebbly sandstone bodies interpreted as ephemeral braided channel deposits the massive laminated planner cross bedded fine to medium grained sandstone bodies interlayered within mudstone reflect sheet flood deposits the mudrocks associated with paleosols represent distal floodplain deposits the shallow lacustrine facies 15 m thick is made up of an alternation of marlstone micritic limestone dolostone and mudrock beds with charophytes and small gastropods both the alluvial and lacustrine facies show evidence of macro and micro pedogenic features pollen assemblages stable 18 o and 13 c isotopes and paleopedogenic features reflect prevalence of arid to semi arid climatic conditions during the bartonian the sequence stratigraphic framework shows an overall fining upward depositional sequence consisting of low and high accommodation systems tracts last hast and is bounded by two sequence boundaries sb 1 sb 2 conglomerate and pebbly sandstone deposits braided channel and sheet flood deposits of the lower part of the alluvial facies reflect a last mudrock and silty claystone facies distal floodplain deposits of the upper part of alluvial facies and its overlying lacustrine facies correspond to a hast the last hast and sb were formed during different accommodation to sediment supply a s ratio phases the variation in a s ratios was mainly controlled by sea level change as well as by local tectonic subsidence and uplift of the basin coincident with the reactivation of the syrian arc system during the bartonian

petrographic and geochemical evidence for the formation of primary bacterially induced lacustrine dolomite la roda white earth pliocene central spain upper pliocene dolomites white earth from la roda spain offer a good opportunity to evaluate the process of dolomite formation in lakes the relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations the la roda mg carbonates dolomite unit occur as a 3 5 to 4 m thick package of poorly indurated white massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds the massive dolomite beds consist mainly of loosely packed 1 to 2 m sized aggregates of dolomite crystals exhibiting poorly developed faces which usually results in a subrounded morphology of the crystals minute rhombs of dolomite are sparse within the aggregates both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites in addition aggregates of euhedral dolomite crystals are usually present in some more clayey sepiolite interbeds the thin porous carbonate mostly dolomite beds exhibit both euhedral and subrounded bacterially induced dolomite crystals the carbonate is mainly ca dolomite 51 54 mol caco3 showing a low degree of ordering degree of ordering ranges from 0 27 to 0 48 calcite is present as a subordinate mineral in some samples sr mn and fe contents show very low correlation coefficients with mg ca ratios whereas sio2 and k contents are highly correlated 18o and 13c values in dolomites range from 3 07 to 5 40 pdb mean 0 06 1 75 and from 6 34 to 0 39 pdb mean 3 55 1 33 respectively samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18o and 13c between the two minerals the correlation coefficient between 18o and 13c for dolomite is extremely low and negative r 0 05 whereas it is higher and positive r 0 47 for calcite the lacustrine dolomite deposit from la roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow hydrologically closed perennial lake the lake was supplied by highly saturated hco3 co32 groundwater that leached dolomitic mesozoic formations precipitation of dolomite from alkaline lake waters took place under a semi arid to arid climate however according to our isotopic data strong evaporative conditions were not required for the formation of the la roda dolomite a significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence

sedimentology and isotope geochemistry of lacustrine carbonates of the oligocene campins basin north east spain the non marine campins basin developed in the oligocene during a period of early rifting of the catalan coastal ranges lacustrine deposits interbedded between two alluvial units comprise shallow and deep lacustrine facies the lower shallow lacustrine facies are made up of microbialite buildups and thin limestone beds in the studied area these facies are overlain by deep lacustrine facies which consist of alternations of several metre thick carbonate and mudstone dominated intervals in addition to calcite which is characteristic of the shallow lacustrine facies aragonite and abundant dolomite are present in the deep lacustrine facies this mineralogical change in the sequence reflects an overall increase in the mg ca ratio of the lake waters the deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the mg ca ratio of the water probably related to variations in the evaporation precipitation rate the sedimentological mineralogical and isotopic characteristics of the campins basin dolomites suggest that in general they are primary in origin the stable isotope data show an approximate covariance between 13c and 18o in the lower shallow lacustrine carbonates calcite which suggests that they formed during the onset of closure of the lake the 13c and 18o values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend the sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters the change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence

microbial primary dolomite from a norian carbonate platform northern calabria southern italy the origin of fine grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the norian dolomia principale of northern calabria southern italy microbial carbonates there consist of stromatolites thrombolites and aphanitic dolomites high relief thrombolites and stromatolites characterize sub tidal facies and low relief and planar stromatolites with local oncoids typify the inter supratidal facies skeletal remains are very rare in the latter whereas a relatively rich biota of skeletal cyanophycea red algae and foraminifera is present in the sub tidal facies some 75 of the succession consists of fabric preserving dolomite especially within the microbial facies whereas the rest is composed of coarse dolomite with little fabric preservation three end members of dolomite replacement fabric are distinguished type 1 and type 2 fabric retentive with crystal size 5 and 5 60 m respectively and type 3 fabric destructive with larger crystals from 60 to several hundred microns in addition there are dolomite cements precipitated in the central parts of primary cavities during later diagenesis microbialite textures in stromatolites are generally composed of thin dark micritic laminae of type 1 dolomite alternating with thicker lighter coloured laminae of the coarser type 2 dolomite thrombolites are composed of dark micritic clotted fabrics with peloids composed of type 1 dolomite surrounded by coarser type 2 dolomite marine fibrous cement crusts are also present now composed of type 2 dolomite scanning electron microscope observations of the organic rich micritic laminae and clots of the inter supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains these probably derived from the fossilization of micron sized coccoid bacteria and spheroidal ovoidal nanometre scale dwarf type bacterial forms furthermore there are traces of degraded organic matter probably also of bacterial origin the microbial dolomites were precipitated in a hypersaline environment most likely through evaporative dolomitization as suggested by the excess ca in the dolomites the small crystal size and the positive 18o values the occurrence of fossilized bacteria and organic matter in the fabric preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn sedimentary dolomite

nanometer scale features in dolomite from pennsylvanian rocks paradox basin utah abstract scanning electron microscopy reveals an association between early dolomite in the pennsylvanian desert creek paradox fm and small approximately 0 1 m nanometer scale textures termed nannobacteria three diagenetically distinct dolomites are present early dolomite limpid dolomite and baroque dolomite in this study only the early dolomite contained nanometer scale features these textures occur as discrete balls and rods clumps of balls and chains of balls precipitation experiments demonstrate that these textures may be the result of precipitation in an organic rich micro environment the presence of these nanometer scale textures in pennsylvanian rocks suggests that these early dolomites precipitated in organic rich bacterial environments

a new location of holocene dolomite formation lake hayward western australia holocene dolomite forms in the sediment of lake hayward a small permanent hypersaline lake in the clifton preston lakeland system western australia the geomorphological setting of dolomite formation in lake hayward is similar to the coorong region in south australia unlike in the coorong region dolomite in lake hayward does not form as a direct precipitate from the lake water but is of diagenetic origin this can be deduced from the following features 1 the dolomite occurs only below 60 70 cm from the sediment water interface 2 dolomite occurs as luminescing cement and 3 dolomite has pristine well formed rhomb shaped crystals the source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite calcite precipitation

biotic and abiotic processes in the formation and diagenesis of permian dolomitic stromatolites zechstein group ne england abstract the crinkly bed of the upper permian zechstein group succession roker formation hauptdolomit z2c in northeast england is a distinctive 1 4 meter thick unit that has been interpreted as a stromatolite of microbial origin or alternatively as the result of purely abiotic precipitation close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the zechstein basin the internal structure of the crinkly bed consists of fine millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt size peloids with rare hollow spheroids the lamination is extremely regular in thickness and laterally persistent for up to 4 meters a microbial community composed of coccoid and filamentous microorganisms inferred from fossil evidence colonized the original sediment surface it is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae the peloids are consequently interpreted as autochthonous this interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite which constitutes both the continuous micritic laminae and the peloids themselves primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters mediated by bacterial metabolism similar to the process that operates in modern intertidal microbial mats the crinkly bed shows macro structures that vary along a continuum from symmetrical to asymmetrical centimeter scale ripple like structures some showing interference patterns through to domical and conical structures more typical of stromatolites the more ripple like structures have a lamination of microbial origin identical to that within the domal conical stromatolites there is no internal cross lamination their origin is equivocal they could be the result of physical current or wave induced deformation of a surficial biofilm or their form could be inherited from true mechanically deposited ripples not observed that provided a template for biofilm growth changes in the nature and strength of the wave and or current regime and possibly in the community structure of the microbial mat produced this array of microbially induced macrostructures miss the latter plus the absence of desiccation features indicative of intertidal to supratidal deposition suggest subtidal sedimentation at a depth of several meters intergranular gypsum cement precipitated during early diagenesis and ferroan sparry dolomite precipitated during burial tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre existing dolomite probably via the biological mediation of heterotrophic bacteria the interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record

δ13c δ18c covariance as a paleohydrological indicator for closed basin lakes the relationship between 18o and 13c in a closed basin lake is a function of hydrological change vapor exchange lake productivity and total co2 or carbonate alkalinity concentration covariance of 18o and 13c usually occurs in lake sediments under the condition of hydrological closure for time periods of the order of 5000 yr or longer on shorter time scales however certain subtleties in the use of the 13c 18o covariance as a hydrology history indicator must be recognized the covariant trend may not be found in hyper alkaline lakes because of the insensitivity of 13c to lake volume changes for less alkaline closed basin lakes covariance of 13c 18o will result from relatively rapid increase or decrease of lake volume with the rapid decrease condition giving rise to heavy and narrow ranged isotopic values for both 13c and 18o due to the coupled evaporation productivity effect when the lake volume remains stable as is the case for an over flowing open lake poor 13c 18o covariance will ensue due to the effect of vapor exchange with the atmosphere we have used the measured 13c 18o covariance in carbonate sediments of mono lake california to deduce paleo hydrological and lake water alkalinity and salinity variations and found the results in consonance with holocene lake level history derived from the 18o and other information

sedimentology mineralogy and isotopic analysis of pellet lake coorong region south australia gravity cores of holocene sediments from a shallow ephemeral lake in the coorong region pellet lake southeastern coastal australia show a mineral assemblage and sequence particular to its hydrology the mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core i e organic rich aragonite to magnesite hydromagnesite aragonite followed by a relative decrease in salinity i e magnesite aragonite hydrornagnesite to aragonite hydromagnesite in the upper portions of the core this sequence is capped by 0 4 m of micritic dolomite and minor amounts of hydromagnesite with the relative abundance of dolomite increasing upwards three stratigraphically and spatially distinct dolomite units upper lower and margin are recognized using stable carbon and oxygen isotope data unit cell calculations and mgc03 mole per cent data of the dolomite detailed x ray diffraction xrd analyses of samples with more than 80 dolomite shows that the dolomite is ordered average unit cell parameters calculated from the xrd patterns indicate that the upper dolomite unit has crystal lattices expanded in the c direction c 16 09 a relative to ideal dolomite c 16 02 a and contracted in the a direction a 4 796 a relative to ideal dolomite a 4 812 a the mol fraction of mgc03 in the upper dolomite shows up to 4 0 f 2 0 mole per cent excess mg in the dolomite crystal lattice calculated from xrd this unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated mg ca ratios transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution the modulated microstructure found in calcium rich dolomite is completely lacking dolomite ordering reflections are present in electron diffraction patterns but are weak stable oxygen and carbon isotope values of the upper dolomite are tightly grouped ave 6l80 x 7 55 6i3c 4 10 yet show three upward lightening oxygen cycles the oxygen cycles correlate with three upward decreases in the calculated mg content of the dolomite zone these cycles may indicate the increased importance of rain water dilution of the brine at times when the water in the lake was at its shallowest levels analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit the lower dolomite is close to stoichiometric has less evaporitic stable isotope values than the upper dolomite and has only a slightly expanded c axis the margin dolomite is ca rich has a more homogeneous microstructure and has expanded a and c axes the abundance of relatively soluble mg bearing phases such as hydromagnesite and magnesite may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment this process may leave a finely laminated dolomicrite deposit which retains little if any evidence of evaporite minerals

stable isotope characterization of the miocene lacustrine systems of los monegros ebro basin spain palaeogeographic and palaeoclimatic implications the lacustrine carbonate facies from three allostratigraphic units n1 n2 and n3 of miocene age in the los monegros region ebro basin are characterized isotopically the dominant facies are marls laminated limestones stromatolitic limestones massive limestones and bioturbated limestones these are associated with lacustrine sulphate deposits and distal alluvial facies palaeohydrological reconstructions of the study area are supported by the isotopic and the mineralogical composition of the carbonates massive and bioturbated limestones occupy a very similar compositional domain and have the lowest isotopic compositions of all the facies 9 18opdb 4 5 and 6 4 13cpdb 0 4 reflecting a short residence time of the water and variable commonly high influence of biogenic co2 laminated limestones and stromatolites define a comparatively enriched domain with 18o values ranging from 6 to 0 and 13c values ranging from 3 5 to 0 these values indicate stronger evaporation and enhanced 12c assimilation due to intense biological activity during periods of longer water residence time marls have isotopic values intermediate between these two domains and correspond to periods of lacustrine dilution dolomite bearing samples mainly laminated and stromatolitic facies show a 18odol cal 7 5 with respect to calcite from the same facies while 13c values show very little enrichment up to 1 the positive correlation r 0 85 between 18o and dolomite contents suggests that dolomite resulted from the progressive evaporative concentration of a single water mass and not from the mixing of waters of different compositions the dolomite is thought to be primary or at most the product of very early diagenetic processes the isotopic composition of the facies and their spatial and temporal variations depend on the depositional environment and were constrained by changing regional paleogeography isotopic trends from units n1 to n3 upper agenian vallesian period display a regular depletion in heavy isotopes that indicates a climatic change towards wetter conditions

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

radium 228 distribution and mixing in san nicolas and san pedro basins southern california borderland sediment profiles of 228ra and 232th and water column profiles of 228ra are presented for san nicolas and san pedro basins located in the southern california borderland these data are used to estimate fluxes of 228ra from the sediments the vertical eddy diffusivity kz in the water column and the upwelling velocity wz the fluxes of 228ra from sediments estimated by integrating the downcore deficiency of 228ra with respect to its parent nuclide 232th are 335 115 atoms m2s for san nicolas basin and 166 50 atoms m2s for san pedro basin san nicolas basin sediments have an excess of 228ra in the upper 4 cm but irrigation and diffusion create a 228ra deficiency from 4 to 30 cm san pedro basin sediments are anoxic and laminated have a 228ra deficiency above 6 12 cm depth and have no surface excess of 228ra basin waters appear to be well mixed horizontally below sill depth and in steady state with respect to 228ra concentrations of 228ra increase from sill depth to the bottom ranging from approximately 1 0 dpm 100 kg to 2 4 dpm 100 kg in san nicolas basin and from 1 6 dpm 100 kg to 3 1 dpm 100 kg in san pedro basin estimates of kz and wz at several horizons in these basins are made by evaluating two steady state mass balance equations one for 228ra and one for heat this approach is hindered by an inability to accurately define the temperature in waters flowing over the sills and the apparent failure of topography to completely restrict horizontal exchange in the upper part of the water column below sill depth in each basin upper limits for kz are 23 cm2 s at 1700 m in san nicolas and 2 8 cm2 s at 850 m in san pedro basin our best estimates for kz and wz for the deeper part of the basins are kz 5 17 cm2 s and wz 50 150 m y in san nicolas basin and kz 1 3 cm2 s and wz 20 50 m y in san pedro basin the differences in kz between basins reflect differences in the buoyancy gradients upwelling velocities increase from the bottom to the sill of each basin

formation of dolomite in the coorong region south australia abstract cores of unlithified holocene sediment from shallow ephemeral lakes in the coorong region s a contain dolomite at various stratigraphic intervals two distinct dolomite types are recognized using stable carbon and oxygen isotope data unit cell calculations transmission electron microscopy tem and bulk composition data detailed x ray diffraction xrd analyses show that the dolomite is ordered but that ordering is variable average unit cell parameters indicate that the crystal lattice of one type of dolomite type a is expanded in the co direction c o 16 08 a and contracted in the ao direction a o 4 799 a relative to ideal dolomite c 0 16 02 a and a o 4 812 a respectively type a dolomite occurs in the centers of the larger basins the mole fraction of mgco3 is as much as 53 1 the unusual crystal chemistry and tem data indicating a heterogeneous microstructure suggest that this dolomite is generated by rapid precipitation from brines which have highly elevated mg ca ratios dolomite ordering reflections are present in electron diffraction patterns but are weak stable oxygen and carbon isotope values are tightly grouped ave 18 o 7 6 s d 0 6 13 c 3 5 s d 0 7 the other type of dolomite type b is ca rich has lower stable isotope values ave 18 o 6 4 13 c 1 2 than type a dolomite has a more homogeneous microstructure is expanded in both the ao and co axes and is generally better ordered than type a dolomite these data suggest type b dolomite is precipitated more slowly from less evaporitic brines than type a dolomite

diagenetic transformation of dolomite into stevensite in lacustrine sediments from jbel rhassoul morocco abstract stevensite in the tertiary lacustrine basin of the jbel rhassoul in morocco is used as fuller s earth observations of the structure and textures of the deposits provide evidence of diagenetic replacement of sedimentary lacustrine dolomite by stevensite carbon and oxygen isotopic compositions confirm the precipitation of primary sedimentary dolomite and the diagenetic formation of secondary stevensite according to thermodynamic calculations stevensite is stable at the expense of dolomite in a silica rich and co2 poor environment under such conditions destabilized detrital illite chlorite and palygorskite release al fe li and trace elements which are incorporated into the structure of stevensite compared with other lacustrine stevensite occurrences the jbel rhassoul deposit seems to correspond to a new mode for the formation of diagenetic stevensite after dolomite

modern dolomite deposition in continental saline lakes western victoria australia microcrystalline dolomite forms a major constituent of holocene sediments of numerous continental saline playa lakes in southeastern australia the lake waters are highly supersaturated with respect to dolomite as well as other mg carbonates but undersaturated or near saturation with respect to calcite and aragonite the dolomite shows no replacement textures and most likely formed by direct precipitation conditions in these lakes that appear conducive to the precipitation of dolomite are 1 high salinity 2 high mg ca ratios and 3 high alkalinity the moderate sulfate levels of the brines do not seem to be inhibiting carbonate precipitation

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

fluid inclusion analysis of twinned selenite gypsum beds from the miocene of the madrid basin spain implication on dolomite bioformation this research work is centred on continental lacustrine gypsum deposits of miocene age cropping out in the easternmost part of the madrid basin these gypsum deposits accumulated in a continental saline lake are characterized by a spectacular distinctive christmas tree morphology and a peculiar dolomite replacement a combination of microscopic petrography and scanning electron microscopy and analytical techniques fluid inclusion microthermometry x ray energy dispersive spectroscopy and x ray diffractometry was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum the objectives were to characterize the continental brine from which the mineral precipitated and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid lake water of low to moderate total salinity between 20 and 90 g l nacl secondary fluid inclusions represent interstitial lake brine in contact with gypsum slightly enriched in total salt content as crystal formation proceeded textural ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes

early diagenetic recrystallization of holocene 3000 years old peritidal dolomites ambergris cay belize three peritidal carbonate crusts and associated intercrust sediments total thickness of 30cm aged 3000 years bp on ambergris cay belize contain 32 100 calcian dolomite x 72 5 dolomite ranging in composition from 40 to 46 mol mgco3 x 43 3 dolomite replaced high mg calcite foraminiferal muds penecontemporaneously with sedimentation forming partially dolomitized sediments and lithified crusts dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth 0 4 m near the top of the section to 1 0 m near the base of the dolomitized section this size increase is not accompanied by any significant decrease in porosity crystal size distributions appear to be log normal and become increasingly broad and flat with depth rietveld x ray pattern fitting structure refinements indicate increasing ca and mg concentrations on their respective sites cation ordering as a function of increasing depth most of the ordering occurs within the first 15 cm of the surface stoichiometry does not increase with depth indicating no relationship between the ca mg ratio and cation ordering strong geochemical trends were observed down section in the dolomite including 1 increasing mn content 44 to 274 ppm and 2 decreasing 13c values 0 9 to 5 5 pdb oxygen isotope values range from 18o 1 3 pdb in the upper part of the section to 2 6 pdb in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend down section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy driven dissolution reprecipitation process ostwald ripening the observed trends in carbon isotopes and mn content probably result from geochemical re equilibration during recrystallization and reflect reducing conditions and an isotopically light organically derived carbon source oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level

biological and sedimentary facies of australian salt lakes abstract four main types of salt lakes are recognized in australian 1 large playa lakes 2 small closed lakes or pans 3 crater lakes and 4 coastal lakes not connected to the sea basically all have an nacl chemistry biological and sedimentary facies characteristic of these salt lakes can be arranged into three basic groups which relate to particular phases in lake regimes these phases are 1 wet phases of ephemeral shallow lakes 2 dry lakes and 3 deep water lakes all elements of the salt lake biota with fossilizable remains are discussed with a detailed account of the palaeo ecological information they offer elements of the nonaquatic biota associated with salt lake sediments are also analysed sedimentary features of australian salt lakes pertinent to palaeoecology are discussed in the context of their association with the biota using numerous illustrations the distribution of the biota on lake floors for example its concentration in particular areas of the lake or its destruction during particular episodes such as lake filling and drying is mentioned in the context of recognizing different facies three major lake regime phases are presented with emphasis on the biological and sedimentary facies which characterize them the paper concludes with a brief discussion of the relevance of recognizing particular salt lake facies and changes in lake histories for the study of palaeoclimates and for mineral and oil exploration

modern non marine dolomite in evaporitic playas of western victoria australia abstract although considerable effort has been made during the past 30 years to understand the occurrence of recent dolomite there are surprisingly few well documented examples of dolomite precipitation in modern continental lacustrine environments fine grained poorly ordered non stoichiometric dolomite occurs in association with magnesite and clay minerals in the modern and holocene sediment of numerous evaporitic lacustrine settings in western victoria we conclude that this dolomite is primary i e nucleated from solution and accumulated as a primary sediment rather than secondary replacement of a precursor carbonate cores from selected basins show that this dolomite micrite has been accumulating for over 10 000 years

a model for the distribution of trace elements between calcite and dolomite abstract a crystal growth model is proposed which allows ions of a trace element to enter the ca and mg sites of dolomite in proportion to the size of the ions relative to that of ca and mg ions and which assigns equal portions of the trace element to the ca site of dolomite and the ca site of associated calcite the model produces calcite dolomite distribution coefficients of 0 79 for mn and 0 43 for fe which may be compared with 0 85 and 0 28 as observed in marble and a distribution coefficient of 2 0 for sr and ba which may be compared with observed values of 2 3 for sr and 1 8 for ba

microdeformation of lacustrine laminite sequences from late miocene formations of se spain an interpretation of loop bedding lacustrine laminated sediments laminites present in late miocene formations of the hijar basin se spain display well developed loop bedding a structure consisting of bundles of laminae that are sharply constricted at intervals giving a morphology of loops or links of a chain the laminite sequences which are interbedded with turbidite marlstones were accumulated on the bottom of a permanently stratified lake developed in a rapidly subsiding basin limited by 010 and 105 normal faults as deduced from both macro and microdeformational analyses the basin evolved under an extensional stress field throughout the late miocene four main types of loops simple and complex loops with subcategories have been recognized within the laminite sequence simple loops of type 1 show the best definite pattern quite similar to pinch and swell structures a type of boudinage typical of stretching of alternating beds where the competence contrast is not strongly marked the remaining loop types display contortion and occasional breakage of laminae microfaulted edges indicative of microdeformation near the boundary between the ductile brittle deformational fields the distribution of the various loop types across the laminite sequence reflects an interplay between progressive lithification of the laminites as sedimentation progressed and tectonic stresses which affected the sediment sequence accordingly a mechanism of deformation under an extensional stress field ultimately related to the creep movement of the main basin faults which resulted in successive seismic shocks of low magnitude is proposed to explain the formation of loop bedding in the laminites

paleoclimatology of the casas del rincon villafranchian series spain from stable isotope data abstract stable isotope analyses were performed on 49 samples of lacustrine carbonate and 69 samples of carbonatic shells of both aquatic and terrestrial gastropods from the pliocene series of casas del rincon jucar river area spain in agreement with the paleontological data the isotope results indicate that the series was deposited under variable climatic and environmental conditions when compared with the paleoclimatic evolution idicated by marine series in western mediterranean area the series of casas del rincon appears to record the marked climatic deterioration which took place about 3 1 3 2 m y ago a chronological succession is proposed for the faunistic events of the studied series the rincon 1 vertebrate fauna is probably 2 5 2 6 m y old

an organogenic origin for widespread dolomite in the cambrian eilean dubh formation northwestern scotland abstract dolomite precipitation from seawater is normally inhibited by the presence of sulfate the low activity of the carbonate ion and the high enthalpy of hydration of the magnesium ion however in modern microbial sediments anoxic degradation of cyanobacterial material by sulfate reducing bacteria can significantly alter ambient water chemistry sulfate is removed and enzymatic breakdown of proteins releases ammonia thereby increasing ph and carbonate alkalinity to levels necessary for dolomite formation magnesium may be supplied from complexed ion pairs in solution and from the breakdown of cyanobacterial sheaths and mucilage in the ancient past where benthic microbial communities dominated the depositional environment it is postulated that large scale shallow subsurface modif cation of ambient waters by sulfate reducing bacteria created conditions favorable to widespread dolomite formation early diagenetic cherts from the cambrian eilean dubh formation of northwestern scotland preserve successive stages of the taphonomic history of cyanobacterial biofabrics in sediments of a continuously aggrading shelf depth related sequential degradation of cyanobacterial mat is accompanied by the appearance and increase in abundance of authigenic dolomite culminating in a crystalline dolomitic fabric with only residual organic clots silicified oolitic grainstone fabrics preserve various ooid morphologies these form a diagenetically modified sequence in which progressive degradation of component microbial material together with leaching of the ooid shell is accompanied by the appearance and increase in abundance of intragranular authigenic dolomite total dissolution of ooids re ulted in the release of a package of crystals as sediment similar sequences are observed in peloids and intraclasts ensuing dolomitic rock fabrics bear no clue to their origin except where grains are preserved in early silicified windows these relationships can be explained by biochemical modification of porewaters during organic diagenesis and provide evidence for widespread authigenic dolomite formation in the shelf sediments this organic degradational model may serve as a basis for understanding the origin of thick platformal dolostone sequences of the proterozoic

modern and holocene carbonate sedimentology of two saline volcanic maar lakes southern australia the basin lakes are two adjacent maar lakes located in the centre of the western volcanic plains district of victoria australia both lakes are saline and alkaline west basin lake is meromictic whereas east basin is a warm monomictic lake the carbonate mineral suite of the modern offshore bottom sediments of these basins consists mainly of dolomite and calcite with smaller amounts of hydromagnesite and magnesite in west basin and monohydrocalcite in east basin the dolomite hydromagnesite magnesite and monohydrocalcite are endogenic in origin being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment water interface the calcite is biologically precipitated as ostracod valves in addition to the carbonates in the modern offshore deep water sediments the lakes also contain a girdle of nearshore carbonate hardgrounds both beachrock and microbialites algal boundstones are present these modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics morphologies and compositions in west basin the hardgrounds are composed mainly of dolomite hydromagnesite and magnesite whereas dolomite and monohydrocalcite dominate the east basin sediments aragonite high mg calcite kutnahorite siderite and protohydromagnesite also occur in these lithified carbonate units stratigraphic variations in the carbonate mineralogy of the holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the basins these changes in conjunction with fluctuations in organic remains and fossil content indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western victoria

recrystallization of dolomite evidence from the monterey formation miocene california dolomites from the upper calcareous siliceous member of the miocene monterey formation exposed west of santa barbara california were analysed for geochemical isotopic and crystallographic variation the data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids recrystallization is recognized by the following compositional and crystallographic variations dolomites have decreasing 18o and 13c compositions decreasing sr contents and increasing mg contents with increasing burial depths and temperatures from east to west in the study area 18o values vary from 5 3 in the east to 5 5 pdb in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west 13c values correlate with 18o and decrease from 13 6 in the east to 8 7 pdb in the west sr concentrations correlate positively with 18o values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west mol mgco3 values inversely correlate with 18o values and increase from a minimum of 41 0 in the east to a maximum of 51 4 in the west rietveld refinements of powder x ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering the fraction of the ca sites in the dolomites that are occupied by ca atoms increases slightly with the approach to stoichiometry the fraction of the mg sites occupied by mg atoms strongly correlates with mol mgco3 even in early diagenetic non stoichiometric dolomites there is little substitution of mg in ca sites during recrystallization the amount of mg substituting for ca in ca sites decreases even further most of the disorder in the least recrystallized non stoichiometric dolomites is related to substitution of excess ca on mg sites

analysis of multiple cosmogenic nuclides constrains laurentide ice sheet history and process on mt mansfield vermont s highest peak abstract constraining glacial history and process on mt mansfield the highest peak in vermont 1339 m a s l provides insight into how the laurentide ice sheet shaped the underlying landscape when latest pleistocene ice retreated and how upland and lowland glacial histories relate here we quantify in situ cosmogenic 10be in 20 bedrock and boulder surfaces as well as in situ cosmogenic 14c in three of those surfaces to assess subglacial erosion and exposure history isotopic concentrations indicate that mt mansfield s lower elevations 400 1200 m a s l were deeply eroded by at least several meters during the last glaciation and then deglaciated rapidly 10be ages across this elevation span are indistinguishable and average 13 9 0 6 ka n 15 suggesting that 800 m of ice thinning occurred within at most about a millennium conversely the higher elevations 1200 m a s l preserve a more complex geomorphic history mt mansfield s summit surfaces contain 10be from previous periods of exposure indicating that the mountaintop landscapes were likely preserved beneath cold based weakly erosive glacial ice exposure ages from the shorter lived isotope 14c are younger 9 7 and 11 7 ka suggesting that mt mansfield s summit was covered until the early holocene perhaps by snowfields ice carapaces and or till our findings in context of previous work suggest that thinning laurentide ice flowed through the valleys for at most hundreds of years following deglaciation of the uplands but that the summit remained shielded by ice or sediment for millennia after the valleys became ice free

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

depositional subenvironments in an arid closed basin the wilkins peak member of the green river formation eocene wyoming u s a the wilkins peak member of the green river formation is a non marine dolomitic deposit that formed in an arid hydrologically closed basin a transect from the margin to the centre of the wilkins peak basin was examined in outcrop exposures in the south eastern corner of the basin and was supplemented with published core descriptions the rocks were divided into seven facies based upon lithology layering style and sedimentary fabrics 1 af facies quartzose conglomerates and sandstones representing sandy alluvial fans coming from the uinta mountains 2 sf facies quartzose peloidal dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans 3 dm facies irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat 4 el facies dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds 5 pl facies finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes 6 sm facies trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology these represent a central salt pan and the surrounding saline mudflat 7 as facies arkosic sandstones siltstones and mudstones which represent braided river and more basinal deltaic plain deposits the five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations there are at least forty 3 4 m thick cycles in the wilkins peak and one large scale cycle comprising the whole member which mimics the facies changes of the smaller cycles this is compatible with the transitions of the wilkins peak member to the tipton and laney members as climatic responses and not shifts in drainage the sheet flood sandstones actively aggrading subaerial mudflats and bedded evaporites of the wilkins peak are apparently unique within the green river formation they reflect the unusual provenance and hydrology of the wilkins peak and suggest chemical and sedimentary models established for the other green river members will not be applicable

mammalian faunas as indicators of environmental and climatic changes in spain during the pliocene quaternary transition abstract the study of mammal communities provides useful knowledge of paleoenvironments and paleoclimates during the quaternary period and better documentation about the main fossil sites is making this task easier paleoecological reconstructions of this study are based on i rodent evolution and species richness ii the cenogram method and iii methods for quantifying climatic parameters these analyses applied to a pliocene quaternary faunal sequence of spain indicate that a climatic change occurred at the end of the pliocene when considerable cooling led to the onset of the glacial interglacial cycles subsequently during the quaternary period alternating environmental patterns occurred with a rather open and arid environment during the cold phases that contrasts with a somewhat more closed and comparatively wet environment during warmer phases these observations are generalized from spain to the remainder of the continent noting that climatic conditions were less harsh and more arid in spain than elsewhere in europe

nannobacteria and the precipitation of carbonate in unusual environments abstract nannobacteria are abundant not only in normal marine limestones but also in other environments such as continental caliches fresh water streams springs water pipes caves etc cells are typically 50 to 200 nm in diameter