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

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

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

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

intra annual oscillations at station aloha north of oahu hawaii inverted echo sounders have been used to make continuous measurements of dynamic height at a site chosen to be representative of the central pacific subtropical gyre these show that the dominant features in dynamic height are fluctuations having peak to peak amplitudes of up to 27 dyn cm and timescales of about 100 days empirical orthogonal function analyses of temperature and salinity profiles suggest that if the oscillations are due to vertical motion within the water column the motion is not that of first baroclinic mode but has much higher vertical wavenumber structure zonal wavenumbers associated with the oscillations indicate westward phase propagation and meridional wavenumbers are close to zero the dominant periods are too short for freely propagating rossby waves two explanations of these observations are that the waves could be doppler shifted by the mean flow or that the region could be an area of active forcing

the influence of weathering processes on riverine magnesium isotopes in a basaltic terrain this study presents major trace element and mg isotope data for the dissolved load and suspended particulates of icelandic rivers draining dominantly basaltic catchments including both glacier fed and direct runoff rivers these samples provide the opportunity to understand the behaviour of mg isotopes during chemical weathering where variations due to lithology are not extant given the significant role of mg in the carbon cycle such variations may provide important information on the regulation of earth s climate hydrothermal waters groundwater precipitation glacial ice basalt glass olivine and representative soils have also been analysed the dissolved load shows a wide range of 26mg compositions compared to the parent basaltic glass 26mg 0 29 ranging from 0 96 to 0 64 while precipitation and hydrothermal waters possess 26mg values of 0 83 and 0 85 respectively with lower mg concentrations than the dissolved load biomass activity in vegetation and organic material in soils and rivers colloids appear to have little effect on the mg isotope compositions rather the data suggest that mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions in some samples seawater in the form of direct precipitation or glacial runoff also appears to be an important source of mg glacier fed rivers groundwaters and some direct runoff rivers with a high ph have higher 26mg than basalt which is most likely due to the incorporation of light mg isotopes in secondary minerals in contrast those direct runoff rivers which have a relatively low ph have low 26mg relative to basalt consistent with preferential incorporation of heavy mg isotopes into secondary phases although it is not possible to rule out some contribution from precipitation riverine suspended particulates are depleted in mobile elements and have 26mg compositions values both higher and lower than unweathered basalt in the glacier fed and direct runoff rivers where the 26mg of the dissolved phase is heavy due to the formation of secondary phases the suspended load is light because it contains more of those phases the opposite is true for the remainder of the direct runoff rivers which have low ph this could be due to dissolution of secondary minerals enriched in light mg which are unstable at low ph or the formation of new secondary phases

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

boron and magnesium isotopic composition of seawater the isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes the isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard given the long residence times of both elements 10 7 years it is commonly assumed that seawater is isotopically homogenous for these systems yet no systematic studies currently exist here we present the b and mg isotopic composition of 26 28 seawater samples from a number of ocean basins that encompass a significant range in salinity 32 to 38 psu temperature 0 3 to 25 9c and water depth 0 to 1240 m we find no significant or systematic variation for either system in accordance with their long residence times we recommend that the mean values we report 11 b 39 61 0 04 2 s e n 28 25 mg 0 43 0 01 2 s e n 26 26 mg 0 82 0 01 2 s e n 26 be used in future studies involving mg and b isotopes copyright 2010 by the american geophysical union

illite smectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the texas gulf coast paleocene eocene abstract mudrock samples from a well penetrating eocene and paleocene strata 1235 4455 m of the texas gulf coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis and the mechanism by which detrital smectitic illite smectite i s is transformed into diagenetic illitic i s idealized smectite and illite end member compositions for interstratified i s are estimated to be k0 00x 10 56mg0 39fe0 57al1 13si3 90o10 oh 2 and k0 53x 10 18mg0 17fe0 16al2 28 si3 40o10 oh 2 respectively there is no obvious difference in the amount of k addition to i s in random and ordered interstratified i s aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite to illite reaction is a complete dissolution precipitation reaction rather than a solid state k 1 al 3 for si 4 substitution reaction depth relat d mineral trends and mass balance calculations suggest that illitic i s may form from kaolinite and possibly illite or mica as well as from smectitic i s although most whole rock element abundances are invariant over the sampled depth interval k2o content increases from ca 2 0 weight percent at 1500 m of burial to ca 3 8 weight percent at 4000 m some of this increase is probably caused by import of k into the shale via fluids derived from interbedded sandstones implying that gulf coast mudrocks behave as open chemical systems during burial diagenesis if this is the case a minimum of 103 pore volumes of fluid must have passed through the most k enriched shales to have introduced the added potassium provided that the k content of present day formation water is representative of that of ancient formation water

using stable mg isotopes to distinguish dolomite formation mechanisms a case study from the peru margin abstract the magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250 m thick sedimentary section drilled into the peru margin during ocean drilling program odp leg 201 site 1230 and leg 112 site 685 previous studies revealed the presence of two types of dolomite type i dolomite forms at 6 m below seafloor mbsf due to an increase in alkalinity associated with anaerobic methane oxidation and type ii dolomite forms at focused sites below 230 mbsf due to episodic inflow of deep sourced fluids into an intense methanogenesis zone the pore fluid 26mg composition becomes progressively enriched in 26mg with depth from values similar to seawater i e 0 8 relative to dsm3 mg reference material in the top few meters below seafloor mbsf to 0 8 0 2 within the sediments located below 100 mbsf type i dolomites have a 26mg of 3 5 and exhibit apparent dolomite pore fluid fractionation factors of about 2 6 consistent with previous studies of dolomite precipitation from seawater in contrast type ii dolomites have 26mg values ranging from 2 5 to 3 0 and are up to 3 6 lighter than the modern pore fluid mg isotope composition the enrichment of pore fluids in 26mg and depletion in total mg concentration below 200 mbsf is likely the result of mg isotope fractionation during dolomite formation the 26mg enrichment of pore fluids in the upper 200 mbsf of the sediment sequence can be attributed to desorption of mg from clay mineral surfaces the obtained results indicate that mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo diagenetic proxy

limestone pseudoconglomerates in the late cambrian gushan and chaomidian formations shandong province china soft sediment deformation induced by storm wave loading this paper focuses on the formative processes of limestone pseudoconglomerates in the gushan and chaomidian formations late cambrian of the north china platform shandong province china the gushan and chaomidian formations consist mainly of limestone and shale marlstone interlayers wackestone to packstone grainstone and microbialite as well as numerous limestone conglomerates seventy three beds of limestone pseudoconglomerate in the gushan and chaomidian formations were analysed based on clast and matrix compositions internal fabric sedimentary structures and bed geometry these pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes i e flat to undulatory disc blade and sheet marlstone and or grainstone matrix and various internal fabrics i e intact thrusted edgewise and disorganized as well as transitional boundaries limestone pseudoconglomerates formed as a result of soft sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates initial deformation i e burial fragmentation liquefaction and injection and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates burial fragmentation resulted from mechanical rupture of cohesive carbonate mud whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials liquefied carbonate sand and water saturated argillaceous mud under increased stress storm wave loading was the most probable deformation mechanism as an external triggering force subsequent re orientation and rounding of clasts were probably prolonged under normal compactional stress eventually disrupted clasts along with matrix materials were transformed into pseudoconglomerates by progressive lithification soft sediment deformation is prevalent in alternate layers of limestone and mud marl stone and or grainstone regardless of their depositional environments

on the significance of δ13c correlations in ancient sediments a graphical analysis of the correlations between c and etoc was introduced by rothman et al 2003 to obtain estimates of the carbon isotopic composition of inputs to the oceans and the organic carbon burial fraction applied to cenozoic data the method agrees with independent estimates but with neoproterozoic data the method yields results that cannot be accommodated with standard models of sedimentary carbon isotope mass balance we explore the sensitivity of the graphical correlation method and find that the variance ratio between c and o is an important control on the correlation of c and e if the variance ratio c o 1 highly correlated arrays very similar to those obtained from the data are produced from independent random variables the neoproterozoic data shows such variance patterns and the regression parameters for the neoproterozoic data are statistically indistinguishable from the randomized model at the 95 confidence interval the projection of the data into c e space cannot distinguish between signal and noise such as post depositional alteration under these circumstances there appears to be no need to invoke unusual carbon cycle dynamics to explain the neoproterozoic c e array the cenozoic data have c o 1 and the c vs e correlation is probably geologically significant but the analyzed sample size is too small to yield statistically significant results

dolomitization in a mixing zone of near seawater composition late pleistocene northeastern yucatan peninsula abstract patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the yucatan peninsula this limestone accumulated during an interglacial high stand of sea level about 200 000 years ago dolomitization was preceded by freshwater diagenesis including precipitation of sparry calcite cement stabilization of mg calcitic skeletal fragments and partial dissolution of aragonitic components this suggests a predolomitization lowering of sea level with the consequent freshening of pore water the subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in mg ca ratio of pore water dolomitization took place during a brief high stand of sea level either shortly after deposition about 200 000 yr bp r more likely about 125 000 yr bp dolomite occurs both as microcrystalline replacement dolomite and as cement the cement is part of the following diagenetic sequence 1 limpid euhedral subhedral calcian dolomite crystals 2 zoned dolomite crystals with zones formed by variations of the calcium magnesium ratio in dolomite 3 layers of alternating calcian dolomite and magnesian calcite or calcite and 4 calcite this sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level average cation composition of the limpid dolomite cement is ca57mg43 electron microprobe analysis zoned cement crystals are composed of ca57 59mg43 41 layers and ca62mg38 layers most of the higher calcium dolomite layers are dissolved forming hollow zone crystals in cement with alternating dolomite and calcite zones the calcite is ca99 97mg1 3 low mg calcite and ca96 93mg4 7 mg calcite the dolomite and mg calcite zones are partially to totally leached 18o compositions of yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75 seawater 25 freshwater to nearly all seawater isotope analyses are for the most stable calcian dolomites more soluble calcium rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite in the cement sequence the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline isotope analyses together with position of dolomite in the cement sequence suggest the most stable calcian dolomite including limpid dolomite precipitated from mixed water with large proportions of seawater and the less stable more calcian dolomite precipitated from fresher mixed water

the composition of the continental river weathering flux deduced from seawater mg isotopes this study establishes the magnesium isotopic composition of seawater and evaluates its constancy as a function of depth and geographic location it also provides results on the magnesium isotopic composition of river water samples draining specific lithologies in combination the results are used to obtain an average magnesium isotopic composition of 1 54 for the global continental weathering flux compared to the 2 value of the global ocean both relative to nist srm 980 analyses of modern and fossil carbonate secreting echinoderms suggest that these can serve as archives of seawater 26mg values it is demonstrated that these records of the magnesium isotopic composition of ancient oceans can be used to make important inferences about the relative contribution of different lithologies to the global continental weathering flux particularly carbonate versus silicate weathering preliminary results suggest that the composition of the continental weathering flux has not undergone dramatic changes during the phanerozoic

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

relation of eustasy to stacking patterns of meter scale carbonate cycles late cambrian u s a abstract an interbasinal study of late cambrian cyclic carbonate successions in the appalachian and cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large scale depositional sequences and the component peritidal to subtidal meter scale cycles that comprise them the focus of this paper is on the small scale cyclicity its probable control by milankovitch forced sea level oscillations and how stacking patterns of meter scale cycles can be used to define internal components of larger scale sequences and estimate variations in relative sea level fining upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening upward subtidal cycles which lack evidence of emergence and form a continuum across the cambrian carbonate platforms eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on late cambrian carbonate platforms evidence for milankovitch forcing of glacio eustatic sea level oscillations is provided by a 4 1 bundling of fifth order meter scale cycles 96 ky within fourth order cycles spanning tens of meters 440 ky within the big horse member of the orr formation in the house range of utah the 4 1 bundling may manifest the short e centricity to long eccentricity ratio of the milankovitch astronomical rhythms systematic changes in the stacking patterns of meter scale cycles can be used in conjunction with fischer plots to define long term sea level cycles on fischer plots of peritidal cyclic successions long term relative sea level rises are characterized by thick subtidal dominated cycles with thin laminite caps long term relative sea level falls are defined by stacks of thin laminite dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand on fischer plots of dominantly subtidal cyclic successions long term sea level rise is characterized by storm dominated open marine carbonate cycles or thick deep ramp shale based cycles falling segments of the fischer plot are characterized by thin shallow subtidal cycles compos d of restricted lithofacies cycle stacking patterns parasequence sets provide the crucial link between the meter scale cycles parasequences and the larger scale sequences and their component systems tracts one and two dimensional models of peritidal and subtidal cycle development indicate that whereas peritidal cycle thickness is primarily controlled by accommodation space deeper subtidal cycle thickness is primarily controlled by sedimentation rate lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm wave reworking that oscillated in harmony with sea level fluctuations and may have acted as a subtidal limit to upward aggradation the 2 d modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses compositions and stacking patterns similar to the late cambrian of north america can be generated by ilankovitch driven composite eustasy

limestone marl alternations a warm water phenomenon ancient limestone marl alternations are concentrated in settings analogous to loci of aragonite accumulation in the modern world they typically occur on shelves in the tropical subtropical climate belt are far more abundant on passive continental margins than on active ones and are rare in upwelling zones in recent studies aragonite was proposed to play an important role in differential diagenesis typical of most limestone marl alternations the coincidence of depositional settings of ancient limestone marl alternations and modern aragonite accumulation is a strong case for this hypothesis if confirmed it could provide a valuable tool for broad scale paleoenvironmental interpretations an additional different type of limestone marl alternations resulted from the cretaceous explosion in productivity of calcitic plankton these pelagic ones are fundamentally different in their style of diagenesis

the mg isotopic composition of cenozoic seawater evidence for a link between mg clays seawater mg ca and climate abstract cooling of earth s climate over the cenozoic has been accompanied by large changes in the magnesium and calcium content of seawater whose origins remain enigmatic the processes that control these changes affect the magnesium isotopic composition of seawater rendering it a useful tool for elucidating the processes that control seawater chemistry on geologic timescales here we present a cenozoic magnesium isotope record of carbonate sediments and use a numerical model of seawater chemistry and the carbon cycle to test hypotheses for the covariation between cenozoic seawater chemistry and climate records are consistent with a 2 3 increase in seawater mg ca and little change in the mg isotopic composition of seawater these observations are best explained by a change in the cycling of mg silicates we propose that mg ca changes were caused by a reduction in removal of mg from seawater in low temperature marine clays though an increase in the weathering of mg silicates cannot be excluded we attribute the reduction in the mg sink in marine clays to changes in ocean temperature directly linking the major element chemistry of seawater to global climate and providing a novel explanation for the covariation of seawater mg ca and climate over the cenozoic

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

behavior of mg isotopes during dedolomitization in the madison aquifer south dakota abstract to constrain how mg isotopes behave during chemical interactions and physical transport in carbonate rich settings we measured 26 mg values of surface water groundwater and dolomite samples from the madison aquifer south dakota groundwater in the madison aquifer chemically evolves by dedolomitization during transport along a 236 km flow path surface streams recharging the aquifer have 26 mg values of 1 08 and 1 18 following recharge groundwater 26 mg values vary between 1 10 and 1 63 up to a distance of 20 km between 20 and 189 km 26 mg values remain nearly constant at 1 40 and a final sample at 236 km shows an increase to 1 09 dolomite exhibits a wide range of 26 mg values between 2 21 and 1 27 reactive transport modeling and isotope mixing calculations employing previously published major ion mass balances 87 sr 86 sr ratios and 44 ca values were used to determine whether dedolomitization reactions namely dolomite dissolution calcite precipitation and mg for na ion exchange fractionate mg isotopes we tentatively attribute the final 26 mg value to preferential uptake of 24 mg during mg for na ion exchange otherwise we find little evidence of isotopic fractionation and observe instead that 26 mg conservatively traces lithologic and hydrologic sources either isotope exchange between dolomite and water with a fractionation factor of 0 or mixing between different water sources establishes the 26 mg value of 1 40 at 20 km this value remains unchanged for the next 169 km of water transport because dolomite adds mg with an average 26 mg value near 1 40 and no other processes cause fractionation calcite precipitation is unimportant either because calcite is not a significant sink for mg or because mg uptake during calcite precipitation under conditions of chemical equilibrium does not fractionate mg isotopes this study suggests mg isotopes undergo conservative transport in carbonate rich settings where waters are in chemical equilibrium with respect to major sources and sinks of mg

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

the magnesium isotope budget of the modern ocean constraints from riverine magnesium isotope ratios we have measured the magnesium mg isotope ratios in 45 rivers including 16 of the largest rivers in the world covering a range of geologic tectonic and climatic drainage basin environments the range in riverine 26mg is 2 5 half the variation in terrestrial rock although the mg isotopic composition of the source rock is important for small rivers at a global scale lithology is of limited significance for mg isotope ratios in rivers part of the variability at a global scale may be attributed to fractionation in the weathering environment the rivers analysed constitute 30 of the global mg riverine flux to the oceans and a flux weighted mg isotope composition of global runoff has been estimated at 1 09 even taking into account uncertainty this is distinct from seawater at 0 82 this difference arises either from the fractionation of mg isotope ratios in the ocean and or a mg budget which is not at steady state the difference is consistent with fractionation by carbonate precipitation in the simplest steady state scenario where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output mg isotope ratios imply a minimum dolomite mg flux of 9 of the total output mg flux the implications of the distinct isotopic composition of the riverine flux from modern seawater is that the mg isotope ratio of seawater has probably varied outside of current analytical uncertainty mg isotope ratios may therefore provide a valuable new tracer in palaeo oceanography

are buoyancy forces important during the formation of rifted margins summary profiles of crustal thickness across rifted continental margins are examined in an attempt to understand the key observations and controlling parameters crustal stretching factor profiles from rifted continental margins supplemented by isochron data for early seafloor spreading have been used to determine a correlation between strain rate e and stretching factor despite the different methods assumptions and data sources our e relationship for rifted margins is consistent with that observed by newman and white for intracontinental rift basins the e relationship we derive is also consistent with the dynamic models of newman and white which include thermorheological strain hardening and strain softening but omit crustal buoyancy forces generated by lateral crustal thickness variations whilst crustal buoyancy forces are not included in the above dynamic models the e data do not necessarily preclude their importance simple numerical models of buoyancy force evolution show that for the first 30 myr after rifting the thermally derived buoyancy forces within the lithosphere that assist extension are larger than the crustal buoyancy forces that oppose extension this rift push force acts as a positive feedback mechanism is of the order of 3 10 12 nm 1 and dominates over the opposing crustal buoyancy forces immediately after rifting it is therefore clear that the delocalising effects of the crustal buoyancy force are dominant over a restricted range of conditions namely at low strain rate and at long times after rifting histograms of the lateral pressure gradients derived from crustal thinning factors along rifted margins show a dominant peak at 0 05 0 3 kpa m 1 and a significant secondary peak at 0 8 0 3 kpa m 1 the lower lateral pressure gradient peak corresponds to thinned parts of the continental crust which is adjacent to unstretched continental crust and may define the edge of a zone of thermal strain softening independent observations show that narrow margins are associated with rapid strain rates and are consistent with thermal strain softening predicted by thermorheological models however the dominant near zero pressure gradient peak is consistent with the operation of crustal buoyancy force processes during rifting which attempt to remove variations in crustal thickness

differential diagenesis of rhythmic limestone alternations supported by palynological evidence alternating cemented and uncemented fine grained layers from pliocene periplatform carbonates of the great bahama bank have fundamentally different diagenetic features the cemented layers consist predominantly of microspar interpreted as an early marine shallow burial cement the intercalated uncemented softer layers are devoid of microspar cement and exhibit signs of mechanical compac tion precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles the needles in the compacted layers show signs of dissolution suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers the lack of compaction in the cemented layers shows that cementation and hence aragonite dissolution in the adjacent compacted layers took place in the shallow burial realm the dissolved carbonate was trans ported by diffusion to adjacent layers and reprecipitated as calcite cement thereby preventing significant compaction of these layers these processes are not yet complete in the material examined and some aragonite remains in the compacted layers the sedimentary composition of the two rock types compacted and uncompacted is similar indicating a similar precursor sediment for both between 1 5 and 7 times the concentration of palynomorphs oc curs in the compacted layers apparently the result of passive diage netic enrichment although the trigger for diagenetic differentiation has not been determined the pure limestone succession studied here appears to serve as a clay free analog to limestone marl alternations

constraining dolomitization by mg isotopes a case study from partially dolomitized limestones of the middle cambrian xuzhuang formation north china the dolomite problem refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record as well as failure of laboratory inorganic dolomite precipitation at near earth surface temperature novel mg isotope systematics provides a promising tool in resolving the dolomite problem here we develop a protocol to place constraints on the dolomitization process by using mg isotopes in this study we measured mg isotopic compositions 26mg of two batches of partially dolomitized limestone samples from the middle cambrian xuzhuang formation in north china 26mg varies between 0 55 and 3 18 and shows a negative linear correlation with 1 mg suggesting that 26mg can be described by a binary mixing between the calcite and dolomite components mg isotopic composition of the dolomite component 26mgdol for the lower sample set that is collected from a 4 m stratigraphic interval containing three high frequency ribbon rock packstone cycles is 1 6 while 26mgdol for the upper sample set from a thick sequence of ribbon rock is significantly higher 0 3 however neither mineralogical and elemental compositions carbon and oxygen isotopes nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples 26mgdol of the xuzhuang limestone is simulated by the advective flow af and the diffusion advection reaction dar models the af model assumes that mg is transported by advective fluid flows while the dar model simulates a contemporaneous seawater dolomitization process in which mg is delivered by diffusion the af modeling result indicates that 26mg of the dolomitization fluid is 0 4 and 1 7 for the lower and upper sample sets respectively these values are significantly higher than modern and cenozoic seawater mg isotopic composition suggesting that the dolomitization fluid is not contemporaneous seawater the af model also predicts spatially heterogeneous 26mgdol with progressive enrichment in 26mg along the fluid flow pathway in the dar model both dolomite content and 26mgdol of the lower sample set can be simulated by using seawater mg isotopic composition of 0 75 thus contemporaneous seawater dolomitization may explain 26mgdol of the xuzhuang limestone furthermore the dar model demonstrates spatially homogeneous 26mgdol to differentiate the af and dar models samples from multiple sections are required nevertheless this study implies that mg isotope might be a useful tool in the study of dolomitization

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

diffusion and mass balance of mg during early dolomite formation monterey formation abstract diffusion from overlying seawater is capable of supplying enough mg to account for all of the early diagenetic dolomite observed in the monterey formation 0 5 20 vol the amount formed depends on the mg flux magnitude into the sediment which is proportional to the rate of dolomite precipitation and inversely proportional to the sediment accumulation rate the amount of dolomite formed is calculated for accumulation rates of 50 and 600 m ma using reaction rate constants obtained from the mg concentration profile of dsdp sites 479 533 and 147 and proposed profiles that assume significant dolomite formation in the uppermost tens of meters of sediment advective pore water flow from compaction decreases the mg flux and becomes increasingly significant as the accumulation rate increases and the reaction rate decreases advection alone during compaction supplies sufficient mg to account for a realistic maximum of 2 vol dolostone within a compacted section an approximate evaluation of the rayleigh number suggests that the low permeability of siliceous ooze prevents convective fluid flow at shallow burial depths the mg mass balance is calculated from dolomite abundance in monterey formation sections santa maria basin area california pore water and solid particle velocities are calculated as a function of sediment depth using a porosity depth relation for average siliceous sediment based on a composite of dsdp data the bulk sediment diffusion coefficient increases with sediment depth at a rate proportional to the geothermal gradient at shallow burial depths seawater is probably the only significant source of mg dolomite volcanic ash and clay minerals are the only significant sinks clay mineral organic matter and silica diagenesis may provide additional mg at deeper burial depths the more or less regular spacing of dolomitic layers may be controlled by the residence time of the dolomite forming layer within the uppermost sediment where the mg flux is a maximum

the effects of diagenesis and dolomitization on ca and mg isotopes in marine platform carbonates implications for the geochemical cycles of ca and mg abstract the ca mg o and c isotopic and trace elemental compositions of marine limestones and dolostones from odp site 1196a which range in depth 58 to 627 mbsf and in depositional age 5 and 23 ma are presented the objectives of the study are to explore the potential for non traditional isotope systems to fingerprint diagenesis to quantify the extent to which geochemical proxies are altered during diagenesis and to investigate the importance of diagenesis within the global ca and mg geochemical cycles the data suggest that ca which has a relatively high solid to fluid mass ratio can be isotopically altered during diagenesis in addition the alteration of ca correlates with the alteration of mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks bulk carbonate 44 ca values vary between 0 60 and 1 31 srm 915a scale the average limestone 44 ca is 0 97 0 24 1sd identical within error to the average dolostone 1 03 0 15 1sd magnesium isotopic compositions 26 mg dsm 3 scale range between 2 59 and 3 91 and limestones 3 60 0 25 and dolostones 2 68 0 07 are isotopically distinct carbon isotopic compositions 13 c pdb scale vary between 0 86 and 2 47 with average limestone 1 96 0 31 marginally offset relative to average dolostone 1 68 0 57 the oxygen isotopic compositions 18 o pdb scale of limestones 1 22 0 94 are substantially lower than the dolostones measured 2 72 1 07 the isotopic data from 1196a suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories numerical modeling supports the contention that such trends can be interpreted as diagenetic and suggests that the appropriate distribution coefficient k mg associated with limestone diagenesis is 1 to 5 10 3 distinctly lower than those values 0 015 reported in laboratory studies with respect to mg isotopes the modeling also suggest that diagenetic fractionation factors of 0 9955 4 5 and 0 9980 2 are appropriate for limestone diagenesis and dolomitization respectively

experimental calibration of mg isotope fractionation between dolomite and aqueous solution and its geological implications abstract hydrothermal experiments at 220 160 and 130 c were performed to calibrate the mg isotope fractionation factor between dolomite and aqueous mg hydrothermal experiments included synthesis of dolomite using different starting materials as well as exchange experiments that used poorly ordered proto dolomite the morphology of synthesized dolomite was dependent on starting mineralogy suggesting that dolomite was synthesized by different pathways hydrothermally synthesized dolomite was initially fine grained disordered or poorly ordered dolomite that with time recrystallized to coarser grained ordered dolomite isotopic exchange was monitored using 87 sr 86 sr ratios and 25 mg tracers and these indicated near complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments the mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology suggesting attainment of isotopic equilibrium combining results from synthesis and exchange experiments the temperature dependent mg isotope fractionation factor for ordered dolomite is 26 mg dolo aq 0 1554 0 0096 10 6 t 2 where t is in kelvin in contrast poorly ordered dolomite has a 26 mg dolo aq fractionation factor that is up to 0 25 lower than that of ordered dolomite and this is attributed to longer mg o bonds in imperfectly ordered dolomite the experimentally calibrated 26 mg dolo aq fractionation factors lie between those calculated by schauble 2011 and rustad et al 2010 the 26 mg dolo aq fractionation factor extrapolated to lower temperatures using the 26 mg t function of this study matches the 26 mg dolo aq fractionation factor obtained by modeling of mg isotope compositions of odp drill core samples this study shows that significant mg isotope fractionation occurs during dolomite precipitation these results collectively demonstrate that mg isotopes in dolomite are a useful tool for studying mg global cycling and dolomitization

dolomitization of atolls by sea water convection flow test of a hypothesis at niue south pacific the core of the former atoll on niue island south pacific is extensively dolomitized a detailed stable isotope study reveals that the dolomites are uniformly enriched in 18o and 13c 18o 2 8 0 5 13c 2 0 0 3 relative to their carbonate precursor the sr isotope ratios are equally constant with depth yielding 87sr 86sr values between 0 70912 and 0 70916 the observed isotopic compositions point to sea water as the dominant dolomitizing fluid the sr age of the dolomites is younger than the biostratigraphic age mid to late miocene placing the dolomitization event at plio pleistocene time a model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water sea water is drawn through the atoll margin and transferred upward by convective flow delivering mg to the sites of dolomite precipitation transported with the sea water are volcano derived metals fe cu zn mn as evidenced by the chemical gradient in

dissolved sulfide catalyzed precipitation of disordered dolomite implications for the formation mechanism of sedimentary dolomite abstract dolomite is a common mineral in the rock record however the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under normal earth surface conditions result in the long standing dolomite problem in sedimentary geology some modern dolomites are associated with sediments where microbial sulfate reduction is active however the role of sulfate reducing bacteria in dolomite formation is still under debate in this study we tested the effect of dissolved sulfide on the precipitation of ca mg carbonates which has been never explored before although dissolved sulfide is one of the major products of microbial sulfate reduction our results demonstrated that dissolved sulfide with a concentration of as low as several millimoles can enhance the mg2 incorporation into the calcitic structure and promote the crystallization of high magnesian calcite and disordered dolomite we also conducted seeded precipitation in experimental solutions containing dissolved sulfide which showed that calcite seeds can inhibit the precipitation of aragonite and monohydrocalcite caco3 h2o and induce more mg2 incorporation we propose that accumulated dissolved sulfide in pore waters in organic rich sediments may trigger the precipitation of disordered dolomite which can be considered as a precursor of some sedimentary dolomite our adsorption experiments revealed a strong adsorption of dissolved sulfide onto calcite faces we suggest that adsorbed dissolved sulfide can lower the energy barrier to the dehydration of mg2 water complexes on the growing carbonate surfaces this study sheds new light on understanding the role of sulfate reducing bacteria in dolomite formation and the formation mechanism of sedimentary dolomite

magnesium isotope and ree compositions of lower ordovician carbonates from eastern laurentia implications for the origin of dolomites and limestones abstract carbonates representing different depositional and diagenetic settings near surface to deep burial including modern sabkha were collected from the lower ordovician st george group in eastern laurentia western newfoundland canada based on petrographic examination three dolomite phases were identified d1 dolomicrite crystals ranging from 4 to 50 m d2 dolomite 50 to 150 m and d3 saddle dolomite 500 m they occur as replacements and cements and exhibit dull for d1 and d3 to zoned for d2 luminescence the occurrence of near micritic dolomites d1 suggests that dolomitization started at low temperatures under near surface conditions during an early stage of diagenesis whereas microthermometric studies of the d2 and d3 dolomites confirmed their formation at higher temperatures on average of 112 19 c and 153 30 c respectively under deeper burial conditions the d1 dolomicrite yields an average 26 mg value of 1 92 0 30 dsm3 which is slightly more negative than those of the d2 dolomite with 1 75 0 34 dsm3 and d3 saddle dolomite with 1 58 0 31 dsm3 the slightly more positive 26 mg values of the higher temperature dolomites d2 and d3 suggest insignificant kinetic fractionation and slight overprinting by 26 mg enriched diagenetic fluids recirculating in siliciclastic and 26 mg rich crustal rocks under closed to semi closed diagenetic conditions the sr isotope signatures support the formation of d1 dolomicrite in an early diagenetic stage and the ree compositions of the investigated dolomites favor their formation in semi closed to closed diagenetic systems with fluid compositions evolving by circulation through crustal rocks with progressive burial furthermore the mg isotopes suggest that the lime mudstones of the st george group had an algal origin whereas the d2 dolomite was sourced and subsequently altered mostly from the d1 dolomicrites of the succession

mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences guadeloupe magnesium mg stable isotopes are increasingly used as a weathering proxy in soils and rivers but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure a better understanding of the behaviour of mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone here we investigate isotopic variations in 26mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from guadeloupe formed under contrasting climatic conditions soils formed in drier conditions low precipitation contain smectite whereas soils formed under wet conditions high rainfall are characterized by halloysite and fe oxides or kaolinite all clay fractions have mg isotopic compositions 26mg 0 41 to 0 10 similar to or heavier than their parent andesite 26mg 0 47 supporting the preferential incorporation of heavy mg isotopes in secondary mg bearing clay minerals with the first direct measurements on clay fractions soils with lighter mg isotope compositions have greater quantities of exchangeable mg the data support a contribution from sea spray to the exchangeable mg pool correlated to the soilweathering degree this study highlights for the first time that the soil 26mg not only depend on 26mg of the parent rock and on any fractionation that might occur but also on the mg retention on the exchange complex which could in turn be controlled by external inputs such as sea spray