controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

pedogenic origin of dolomite in a basaltic weathering profile kohala peninsula hawaii we document stoichiometric dolomite occurring in a nonsaline quaternary soil on the kohala peninsula northwestern hawaii geologic constraints and geochemical and isotopic data confirm that this dolomite is not the result of marine influence or wind blown dust the strontium isotopic composition of the dolomite 87sr 87sr 0 7045 0 7048 is indicative of its derivation primarily from the weathering of basaltic parent material rather than from meteoric water or seawater infiltration of soil waters with elevated mg ca 1 derived from alteration of ferromagnesian minerals such as olivine likely led to dolomitization of early precipitated soil calcite and or to direct dolomite precipitation in the profile this demonstrates that well ordered dolomite can form in a nonmarine environment at temperatures 100 c without undergoing burial diagenesis

shallow burial dolomitization and dedolomitization of cenozoic cool water limestones southern australia geochemistry and origin abstract the gambier limestone a bryozoan rich cool water carbonate of late eocene to early miocene age in the otway basin of southeastern australia is about one third variably dolomitized limestone and dolostone individual dolomite crystals range from 10 to 500 m and are of three types 1 silt size crystals 4 64 m diameters having a brightly luminescent core 10 20 m of nearly stoichiometric dolomite with 1 mol mnco3 and sio2 but no fe surrounded by a thin dull or alternating dark and bright cl cortex that is nonstoichiometric with 44 46 mol mgco3 but sharp to diffuse zoning in mn and fe 2 sand size crystals which also have a brightly luminescent and stoichiometric core and dark and bright non stoichiometric thin inner cortex but are mostly a thick outer cortex at ca 50 m diameter that is nonluminescent and nonstoichiometric with some fe but no mn and 3 identical sand size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement these three crystal populations represent two distinct dolomites and their alteration product the smallest of the silt size crystals which are rarely preserved are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition the 18o values of dolomite rhombs 1 6 to 2 9 correlate positively with 13c values 0 7 to 3 0 with the lowest values from silt size or small sand size crystals and the highest values from large sand size crystals with leached unfilled cores low and variable 18o and 13c values in conjunction with high and variable mn and fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater higher values for the outer cortex in sand size crystals are typical of those expected for precipitation from normal or near normal seawater low sr mn and fe contents in the largest crystals in conjunction with fluid inclusions of seawater salinity in the outer cortex are also consistent with the bulk of the large crystals having precipitated from seawater using the sr isotope evolution curve for cenozoic seawater sr isotope ratios of the dolomites can be divided into four distinct groups which correspond to times of major transgressions this cenozoic example in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression regression offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained

enigmatic carbonates of the ombombo subgroup otavi fold belt namibia a prelude to extreme cryogenian anoxia abstract the ombombo subgroup of the otavi fold belt kaokoveld namibia preserves a succession of clastic and carbonate sediments with unusual sedimentary features the stratigraphy of these units is discussed here in detail for the first time since their initial definition with particular emphasis on the sedimentology of carbonate units early neoproterozoic shales of the beesvlakte formation equivalent to the zambian katangan copperbelt s lower roan formation host evaporitic lithologies and minor copper mineralisation the overlying dolomitic 760 ma devede formation contains carbonate platformal lithologies which are in many ways dissimilar to phanerozoic shallow water carbonates this includes unusual curl breccias sheet cavities carbonate shrubs and tepee carbonate lithologies which contain large quantities of fibrous cements curl breccias are defined here as distinctive curled intraclasts of laminated dolomite that often have shrinkage cracks in their margins and are cemented by fibrous dolomite cements fibrous cements take on two forms an early length fast fascicular optic dolomite and a later length slow phase with unit extinction the presence of overlying internal sediments the fibrous habit of these first generation cements as well as their preserved cathodoluminescent and optical character suggests that these cements originally precipitated as calcite and dolomite marine cements respectively after this initial marine calcite precipitation all components of devede formation carbonates have been mimetically dolomitised preserving original depositional fabrics combined with the presence of marine dolomite cements this style of dolomitisation is suggested to be syn sedimentary similar to that of some cryogenian dolomites suggesting unusual ocean conditions during the early neoproterozoic in particular the presence of dolomite marine cements which have been linked to ocean anoxia and high seawater mg ca conditions suggests that the onset of marine anoxia in the neoproterozoic may have occurred during the growth of the devede formation carbonate platforms prior to the sturtian glaciation this implies that glaciation may not be the sole cause for the development of marine anoxia during the neoproterozoic it is possible that marine anoxia in this southern african ocean basin may have contributed to a build up of metals in seawater perhaps pre enriching basinal fluids for large scale stratabound copper mineralisation

new geochemical support for mixing zone dolomitization at golden grove barbados dolomite occurs in the golden grove reef terrace of late pleistocene age in southeastern barbados where fore reef lithologies have been partially to completely dolomitized interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade among the points of contention are the timing of deposition dolomitization and dolomite cement precipitation and the relationship of replacement dolomite with dolomite cement data from this study indicate that dolomitization occurred during a single relative sea level fall following reef development at marine oxygen isotope stage 7 3 at approximately 216 ka replacement dolomitization affected both micritic matrix and skeletal grains during or after replacement dolomitization dolomite and calcite cements filled primary and secondary pores the cement paragenesis comprises early dolomite cement complexly banded cements of alternating calcite and dolomite late dolomite cement of near stoichiometric composition and a final phase of blocky calcite cement replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite dolomite cements and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition dolomite oxygen isotope values range from 0 18 to 3 85 pdb and become progressively more depleted throughout the paragenesis indicating a greater influence of meteoric water through time remarkably depleted carbon isotope compositions for both replacement dolomite and dolomite cement 9 53 to 23 04 pdb originated from oxidation of thermogenic methane advected from the underlying accretionary complex methane was oxidized in marine waters that subsequently mixed with meteoric waters as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis dolomite strontium contents are substantially elevated above those expected for marine dolomitization ranging from 640 to 1650 ppm and increasing throughout the paragenesis meteoric water provided the excess strontium as aragonite to calcite transformation in the exposed reef terrace increased the fluid sr ca ratio the continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association these new data provide additional support to the mixing zone interpretation for golden grove and that the model of mixing zone dolomitization remains viable

strontium isotope stratigraphy for the late cretaceous a new curve based on the english chalk marine 87sr 86sr decreases from 0 70775 in the cenomanian to 0 70730 in the middle turonian before increasing in a near linear manner to 0 70775 in the early maastrichtian this variation has been defined using samples from the english chalk that are closely integrated with the macrofossil and microfossil biostratigraphy of northwestern europe with this new isotope curve a stratigraphic resolution is attainable in correlation that is typically 0 8 ma for the santonian and campanian stages isotopic and biostratigraphic correlations between dorset and norfolk in the uk agree within the limit of analytical error in 87sr 86sr authors

strontium isotope correlation of the basal maastrichtian stage in antarctica to the european and us biostratigraphic schemes new 87 sr 86 sr dating allows the correlation of a marker horizon within the prolific late cretaceous gunnarites antarcticus faunal assemblage of the cape lamb member snow hill island formation vega island antarctica with reference sections in europe and the usa this horizon is between 81 5 and 96 5 m above the base of the g antarcticus assemblage replicate analysis of six macrofossils from within it yielded a mean value for 87 sr 86 sr of 0 707 735 9 0 000 004 3 2 s e n 17 this ratio in turn yields a numerical age of 71 0 0 2 ma when compared to the standard 87 sr s6 sr reference curve for the latest cretaceous for which the campanian maastrichtian boundary is placed at 71 3 0 5 ma the value of 0 707 735 9 0 000 004 3 correlates to a level in the belemnella lanceolata belemnite zone of the chalk of northwestern germany that is 2 5 5 m beneath the base of the overlying b pseudobtusa zone and 7 5 m above the campanian maastrichtian belemnite boundary and to an interval within the ammonite zonation of the us western interior that spans the early maastrichtian baculites baculus and b eliasi zones but with a most likely level within the b eliasi zone an earliest maastrichtian age is thereby determined for the antarctic horizon and indeed for the entire g antarcticus assemblage gunnarites sensu lato is an important ammonite marker for the base of the maastrichtian throughout the southern high latitude regions and the associated large heteromorph ammonite diplomoceras may comprise a macrofossil link back to the maastrichtian type sections

formation of dolomite in recent island arc sediments due to gas seawater sediment interaction dolomite occurs as a primary precipitate that cements re cent volcaniclastic sands in the shallow marine environment of luise harbor lihir island papua new guinea field observations and geo chemical data suggest that dolomite precipitates through complex min eral reactions in response to discharge of gas 90 co2 and heated seawater from hydrothermal vents in the seafloor gas discharge caus es the formation of shallow circulation cells and cold seawater is drawn into the sediment where it is heated to approximately 100 8c reaction between hydrothermal co 2 g primary sediment grains and heated seawater causes the precipitation of primary dolomite once temperatures are above 70 8c the mg ca ratio in seawater increases because caso 4 precipitates while at the same time the dissolution of iron oxide minerals increases the ph both reactions favor dolomite precipitation ferrous iron reacts with h 2s to form co precipitated fe sulfide minerals the dolomite is nearly stoichiometric and occurs as zoned inclusion rich and inclusion poor spherulitic cements many of which are present as coalesced composite spherules oxygen isotope values for dolomite of 28 8 to 210 2 vpdb constrain the temper ature of precipitation to be between about 77 to 1108c strontium iso tope ratios for dolomite average 0 70886 are close to those of modern seawater their slightly less radiogenic compositions implicate minor addition of magmatic strontium hydrothermal dolomite may develop in shallow near surface sediments in active tectonic settings

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

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

kinetics of dolomitization dolomitization of caco3 in the laboratory at 175 c demonstrates that the transformation has two distinct stages an induction stage during which no detectable products form and a nucleation growth stage during which dolomite forms at the expense of the reactant caco3 experiments at 218 c demonstrate that as the mg ca ratio of the solution increases the length of the induction stage decreases addition of co2 to the solution decreases the induction stage and also changes the overall transformation mechanism previously published data show that the induction stage is affected by the temperature of the reaction and the surface area and mineralogy of the reactant the induction stage is often the slowest part of the overall transformation characteristics of natural dolomites that are consistent with the experimental results include the following 1 dolomite selectively replaces fine crystalline caco3 2 most carbonate rocks are 100 dolomite or 100 calcite 3 limestone dolomite contacts are usually sharp and involve a decrease in the number of dolomite rhombs but the size of the rhombs remains constant 4 dolomite in completely dolomitized rocks is more stoichiometric than dolomite in partially dolomitized rocks 5 aragonite is selectively dolomitized in modern sabkha sediments and 6 modern dolomite is most abundant in areas of elevated mg ca ratio or hco3

origin of modern dolomite in surface lake sediments on the central and western tibetan plateau abstract modern dolomite was found in the sediments of 20 lakes on the central and western tibetan plateau during lake surveys in august to november of 2012 2015 we analyzed the mineralogy of authigenic carbonates and the chemical composition of lake water to investigate the environmental factors affecting dolomite formation and to determine the mechanisms of dolomite formation x ray diffraction analysis suggested that the dolomite content ranged from 3 to 10 with a mean of 5 and that the majority of the dolomite in the surface lake sediments was ca dolomite dolomite formed in a wide range of salinity environments there was a positive linear relationship between salinity and mg ca ratio whereas the ph and sulfate so42 concentration are not significantly correlated with mg ca ratio high mg ca ratio high salinity high sulfate concentration and high ph are important environmental factors in favor of dolomite precipitation evaporation and microbial activity are potential mechanisms for dolomite formation in the central and western tibetan plateau the positive relationship between 18o and 13c indicates that the dolomite formed in lakes has a long residence time and undergoes continuous evaporation the crystal morphologies of dolomite were scattered euhedral grains with diameters of 2 10 m as well as sub spherical and spherical grains

diffusive cation fluxes in deep sea sediments and insight into the global geochemical cycles of calcium magnesium sodium and potassium the biogeochemical cycles of the ocean s major cations calcium magnesium sodium and potassium are linked to the ocean s alkalinity budget through terrestrial weathering and the subsequent formation and burial of calcium carbonate in the ocean chemical reactions within ocean sediments play a critical role in the biogeochemical cycles of the major cations as indicated by geochemical gradients both positive and negative in the concentrations of these cations within the pore fluid system i e fluid trapped between sediment particles here we review the biogeochemical cycles of calcium magnesium and sodium and provide new estimates of the diffusive fluxes of these cations within marine sediments to explore the importance of these sedimentary processes we quantify these fluxes by compiling a global database of pore fluids from the various ocean drilling programs deep sea drilling program dsdp ocean drilling program odp international ocean drilling program iodp comprising nearly 700 locations which allows a wider geographic coverage and therefore better integrated flux estimates than previous work has allowed the myriad of subseafloor chemical reactions that may influence the concentrations of the major cations in pore fluids include authigenic carbonate precipitation carbonate dissolution clay mineral formation and ion exchange as previous work has shown we confirm that these integrated fluxes are globally significant because the dsdp odp iodp cores begin sampling one meter below the sediment water interface additional studies of the processes within the top meter are needed to accurately calculate total cation fluxes across the sediment water interface delineating the various processes that control the major cation chemistry of seawater over geologic time scales remains critical for understanding the operation of the co2 silicate weathering thermostat on geologic timescales

strontium isotope stratigraphy in the late cretaceous numerical calibration of the sr isotope curve and intercontinental correlation for the campanian the white chalk exposed in quarries at lagerdorf and kronsmoor northwestern germany provides a standard section for the european upper cretaceous the sr 87 sr 86 values of nannofossil chalk and belemnite calcite increase upward through 330 m of section from less than or equal to 0 70746 in the upper santonian to greater than or equal to 0 70777 in the lower maastrichtian the data define three linear trends separated by major points of inflection at stratigraphic heights in the section of 162 m 75 5 ma in the upper campanian galerites vulgaris zone and at 6 m 82 9 ma just above the base of the campanian in the inoceramus lingua goniateuthis quadrata zone the temporal rate of change of sr 87 sr 86 was constant through each of the linear segments of our isotope curve when viewed at the resolution of our average sampling interval 0 15 m y fine structure if rear may record brief 100 kyr excursions of srsr sr 87 sr 86 from values expected from the overall trends in lagerdorf the boundary between the santonian and campanian stages taken here as the level of first occurrence of the belemnite gonioteuthis granulataquadrata has an sr 87 sr 86 of 0 707473 5 this is within error of the values of 0 707457 16 for this boundary in the u s western interior base of the scaphites leei iii zone and 0 707479 9 for this boundary in the english chalk top of the marsupites testudinarius zone in kronsmoor the boundary between the campanian and maastrichtian stages taken here as the level of first occurrence of the belemnite belemnella lanceolata has an sr 87 sr 86 of 0 707723 4 this is within error of the values of 0 707725 20 for this boundary in the u s western interior base of the baculites eliasi zone and 0 707728 5 for this boundary in the english chalk defined as in germany

microbially catalyzed dolomite formation from near surface to burial abstract over the past century a number of models have been developed to explain the vast stratigraphic distribution of authigenic dolomite these models include hydrogeological geochemical and geomicrobiological interpretations that shed light on kinetic constraints on low temperature dolomite formation and in particular how certain microbes can overcome such limitations in this review we focus on the direct and indirect mechanisms by which microbes facilitate the nucleation and shallow burial diagenetic stabilization of dolomite in marine sediments we also consider the likely effects of local and global scale biogeochemical perturbations over the distribution and activities of subsurface microbes and thus potentially over the stratigraphic distribution of dolomite considering the many advances that have been made in the fields of geomicrobiology and biogeochemistry during the past 20 years investigating the important role microbes play in sedimentary systems will undoubtedly contribute to a more complete interpretation of dolomite formation in the rock record

high resolution stratigraphy with strontium isotopes the isotopic ratio of strontium 87 to strontium 86 shows no detectable variation in present day ocean water but changes slowly over millions of years the strontium contained in carbonate shells of marine organisms records the ratio of strontium 87 to strontium 86 of the oceans at the time that the shells form sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium 87 to strontium 86 with a resolution that is similar to that of other techniques used in age correlation this method may prove valuable for many geological paleontological paleooceanographic and geochemical problems

coupled δ44 40ca δ88 86sr and 87sr 86sr geochemistry across the end permian mass extinction event abstract we report high resolution high precision 44 40ca 88 86sr and 87sr 86sr records spanning the permian triassic boundary ptb from the meishan and dajiang carbonate successions in south china the goal of the study was to understand the behavior of ca and sr isotopes during a time period in earth history characterized by severe biological and environmental perturbations including a major mass extinction flood basalt volcanism ocean acidification and sea level fluctuations dajiang displays negative 44 40ca and 88 86sr excursions and invariant 87sr 86sr ratios in the

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

spheroidal dolomites in a visean karst system bacterial induced origin spheroidal dolomite crystals occur in the karstified top of a dinantian dolomite sequence in eastern belgium the spheroidal dolomite crystals are best developed at the base of the karst system the dolomite crystals are characterized by a spherulitic or dumb bell inclusion pattern and are overgrown by dolomite cements with a rhombohedral outline they are considered to be bacterially related precipitates based on 1 textural similarities with documented bacteriogenic precipitates 2 the presence of bacterial microspheres and framboidal pyrite embedded within the dolomite and 3 their general geological setting the geochemical characteristics of the dolomites and associated minerals support a bacterial origin the ubiquity of framboidal pyrite depleted in 34s 34s 22 4 to 25 5 ocdt testifies to a period of bacterial sulphate reduction the isotopic composition of the spheroidal dolomites 13c 2 4 to 3 2 opdb and 18o 3 8 to 3 4 opdb suggest a contribution from oxidized organic carbon produced during bacterial sulphate reduction sulphate reduction may also result in a concomitant 18o depletion if the system is nearly closed it is however evident from the sulphur isotopic composition of associated framboidal pyrite that the system was fairly open the 18o depletion of the spheroidal dolomite crystals 18o 3 8 to 3 4 opdb and their occurrence adjacent to and within karst cavities suggests a mixing zone origin with a significant proportion of freshwater in it the rhombohedral cement overgrowths have calculated 18o values in the range of 0 to 5 3 opdb which reflect precipitation from normal to slightly evaporated contemporaneous seawater

near bottom shear stresses in a small highly stratified estuary 1 this paper examines near bottom shear stresses at two locations in a small highly stratified estuary the navesink river new jersey usa the shear stresses obtained from acoustic doppler current profilers adcps operating in the recently developed mode 11 at a high spatial resolution are examined in detail measurement noise of the adcp is estimated using autocorrelation analyses and is found to be small but correlated with velocity measurement noise in estimates of reynolds stress is found to be somewhat smaller than that obtained in earlier studies for the adcp operating in mode 4 internal waves may have produced observed correlated oscillatory motions the effect of these motions on reynolds stress estimates is quantified and shown to be at least an order of magnitude less than the total measured stress and probably much less the high spatial resolution profiles from the adcp operating in mode 11 allow excellent measurements of near bed vertical velocity gradients log profile estimates of stress are poorly related to reynolds estimates even when stability measures based on near bed density gradients are taken into account we show that upper layer pycnoclines likely inhibit turbulence and bias log profile estimates even at a half meter above the bed where the water column is well mixed

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

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

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

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

oxygen isotope microanalysis by secondary ion mass spectrometry suggests continuous 300 million year history of calcite cementation and dolomitization in the devonian bakken formation calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems unravelling detailed histories and timescales of cementation and replacement is however often compromised by the limited spatial resolution of many analytical techniques in some cases multiple grains are co analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish in this study we have used 10 micrometer resolution in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual 10 50 micrometer size crystals of dolomite and calcite from six samples in a single core of upper devonian middle bakken member siltstones and sandstones the major tight oil formation of the williston basin usa this amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred petrographic data show that microcrystalline calcite cement is an early phase and combined with mineralogical data suggest that much of the dolomite replaces calcite over spatial scales of less than a centimeter analyses of individual calcite crystals have a range of 5 for 18o in the group of crystals and for the group of individual dolomite crystals 10 these sub centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low resolution studies may have inadvertently analyzed mixed phases there is no relationship between dolomite texture and isotopic composition at this spatial scale microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen but cannot be resolved with a 10 micrometer spot size assuming since it is an early cement that calcite precipitated from seawater 18o 1 5 it formed at ca 15 40 c mainly at the lower temperatures present day formation waters in devonian rocks in this region have oxygen isotope compositions of 7 8 vsmow using these values as a likely dolomitizing fluid we suggest that dolomitization occurred continuously between 40 and 140 c over 150 200 million years most likely in a fluid with a high mg ca ratio resulting from gypsum formation in local evaporites we suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of mg in a very sluggish flow regime dolomitization is incomplete because of a limited supply of mg

early miocene subtropical water temperatures in the southeast pacific abstract cenozoic climate of western south america is strongly controlled by features like andean uplift and the humboldt current the first strontium isotope age data from central and southern chile provide a latest oligocene to late early miocene age for classic warm water mollusk faunas reaching as far south as 45 s comparison with the biogeography of congeneric living species indicates that sea surface temperatures off central and southern chile during that time were at least 5 c higher than today i e minimum annual mean sea surface temperatures for darwin s navidad fauna at 34 s are estimated as 20 c as expected the number of tropical taxa decreases towards the south but several are still present as far south as 45 s the ages scatter relatively broadly between 24 and 16 ma partly even within individual localities shallow water and deeper water faunas are revealed to have similar ages when considered in light of convincing micropaleontological evidence for late miocene to early pliocene depositional ages the sr isotope data support a hypothesis that the mollusk fauna is reworked

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

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

mineralogy and mn geochemistry of laboratory synthesized dolomite abstract dolomite was synthesized in hydrothermal bombs to assess influences on mn partitioning into the mg and ca sites calcite was added to magnesium chloride solutions doped with 0 10 20 or 50 ppm mn runs were made for 8 to 241 hr at 192 and 224 c the calcium content of dolomite increased to 60 64 after 75 hr at 192 c and reverted to stoichiometry thereafter increases in the mn content of the starter solution progressively retarded formation of dolomite for dolomites formed without added mn or formed in less than 40 hr with 10 ppm added mn the esr signals were too broad to permit determination of mn site distribution ratios measurable ratios of mn in the mg site to mn in the ca site vary as a linear function of time from 0 3 to 7 for samples prepared at 192 c and from 3 to 16 for those prepared at 224 c stoichiometric dolomites have higher mn partitioning ratios than nonstoichiometric dolomites as observed in natural systems dolomites formed in supratidal sabkha early groundwater and dorag conditions may differ in their mn signature from those formed during burial diagenesis

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

two major cenozoic episodes of phosphogenesis recorded in equatorial pacific seamount deposits seamount phosphorites have been recognized since the 1950s but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits representative samples from equatorial pacific cretaceous seamounts were analyzed for chemical mineralogical and stable isotope compositions the phosphorites occur in a wide variety of forms but most commonly carbonate fluorapatite cfa replaced middle eocene and older carbonate sediment in a deep water environment 1000 m element ratios distinguish seamount phosphorites from continental margin plateau and insular phosphorites uranium and thorium contents are low and total rare earth element ree contents are generally high ree ratios and shale normalized patterns demonstrate that the rees and host cfa were derived from seawater strontium isotopic compositions compared with inferred cenozoic seawater curves define two major episodes of cenozoic phosphatization late eocene early oligocene 39 34 ma and late oligocene early miocene 27 21 ma three minor events are also indicated the major episodes occurred at times of climate transition the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth the paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish fluctuations in climate sealevel and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep sea phosphorus reservoir as temperature gradients in the oceans increased antarctic glaciation expanded and oceanic circulation and upwelling intensified expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts however the bottom waters never became anoxic during the phosphogenic episodes fluctuations in the ccd and lysocline co2 fluxes and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which coupled with increased supplies of dissolved phosphorus promoted replacement processes the late eocene early oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphorite deposits in the geologic record whereas the late oligocene early miocene episode and middle miocene event are matched by large deposits distributed globally the seamount phosphorites are exposed at the surface of the seamounts and have been for most of the neogene and oligocene the phosphorites do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to cfa mass balance calculations indicate that about 5 4 19 1012 g of p2o5 are locked up in equatorial pacific seamount phosphorites that amount is equivalent to about 2 7 years of the present annual input from rivers

strontium isotope stratigraphy of kita daito jima atoll north philippine sea implications for neogene sea level change and tectonic history abstract a detailed chronology of a 432 m core from kita daito jima an atoll on the philippine sea plate has been obtained by strontium isotope stratigraphy the atoll carbonate extends to late oligocene age the top 100 m plio pleistocene section of the core has been dolomitized and dolomite also extends to middle miocene sediments sr isotopes identify two dolomitization events at 2 and 5 ma atoll growth appears to have been continuous between 18 8 and 24 3 ma and the fit of the growth history to a standard model provides a successful confirmation of darwin s subsidence theory the atoll growth appears to have commenced about 42 ma ago 6 ma after volcanism producing a 2 km column of atoll carbonate in agreement with bathymetry the subsidence model defines an apparent loss of 360 m of the sedimentary record since the early middle miocene boundary the amount in excess of the 100 m associated with sea level fall is attributed to plio pleistocene uplift at rates of 20 30 m ma associated with the forebulge of the philippine sea plate prior to its subduction uplift rates have also been estimated using a the heights of dated last interglacial coral reefs on kita daito jima and nearby minami daito jima 17 m ma and 29 m ma b sr isotope ages of calcites from surface samples on minami daito jima 21 m ma and c contrasts in elevation with okino daito jima 100 km away 22 m ma the locations of hiatuses and ages of dolomitization indicate sea level falls of 80 m at 17 16 ma 30 m at 16 15 ma 125 m at 11 ma and 90 m at 5 ma and at 2 ma the data have been combined with those from enewetak atoll to produce a curve of sea level change for the neogene

ordered low temperature dolomite mediated by carboxyl group density of microbial cell walls abundant in the ancient rock record early dolomite remains scarce in modern systems at low temperatures 50c even those systems supersaturated with respect to dolomite this scarcity is attributed to kinetic inhibition including complexation of mg2 by water and sulfate carbonate activity and mg ca ratio recent investigations point to a function for microbial metabolisms and surfaces in which disordered phases are formed here we report the precipitation of primary ordered dolomite at 30c facilitated solely by the cell walls of two nonmetabolizing archaea from saline solutions with an mg ca ratio of 1 1 5 1 and 10 1 and slightly saturated with respect to dolomite control experiments using bacteria and functionalized microspheres did not precipitate dolomite archaeal cell wall functional groups were approximately one order of magnitude higher than the bacteria and spheres used in this study from these results we propose a mechanistic model in which carboxyl groups associated with cell wall biomass and exopolymeric substances dehydrate mg ions further promoting carbonation and leading to dolomite nucleation these data explain reports of low temperature dolomite formation associated with numerous microbial metabolic guilds including bacteria and archaea and those reported in association with exopolymeric substances or cell wall surfaces and identify a key and widespread mechanism in the formation of disordered dolomite and ordered primary phases of dolomite at low temperature importantly the functionalized dead and nonmetabolizing biomass is the key in low temperature dolomite precipitation not active microbial metabolism these observations may lead to new predictive models for the distribution of dolomite

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

strontium isotope stratigraphy for late cretaceous time direct numerical calibration of the sr isotope curve based on the us western interior the 87sr 86sr of sr in macrofossil carbonate from upper cretaceous strata deposited in the western interior seaway usa provides a sr isotope curve for late cretaceous time cenomanian early maastrichtian the curve is calibrated biostratigraphically against the most refined ammonite zonation known for the interval and calibrated numerically with 39ar 40ar dates for 20 bentonites within the sequence marine 87sr 86sr decreased from 0 70743 in the late middle cenomanian to a late cretaceous minimum of 0 70730 in the late turonian 89 ma from the minimum 87sr 86sr increased through a middle campanian inflexion minimum at 77 ma maximum at 78 80 ma to reach 0 70772 at the campanian maastrichtian boundary thereafter 87sr 86sr increased through a very short inflexion in the latest early maastrichtian to the limit of our sampling in the earliest late maastrichtian the inflexions provide global event markers for correlation for most of the period covered by our curve resolution in dating and correlation of 0 8 ma should be achievable with our 2 s d precision in measurement of 18 10 6 our 87sr 86sr data show no evidence of having been affected by influxes of freshwater into the us western interior seaway from rivers including those that drained the sevier orogenic belt

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

sedimentary characteristics and origin of dolomitic ooids of the terminal ediacaran dengying formation at yulin chongqing south china abstract the ediacaran was a crucial transition period in the evolutionary history of the earth ediacaran ooids in the dengying formation from the yulin section in southwestern china are important archives of paleoenvironmental information although these ooids have been totally dolomitized the cortical fabric characteristics are locally preserved well indicating that these ooids were mimetically replaced the common occurrence of concentric radial fabrics and low level of sr concentrations 60 3 8 8 ppm in the studied ooid cortex is more likely suggestive of a primary low mg calcite mineralogy and that a calcite sea might exist during deposition of the second member of the dengying formation in situ geochemical data for the ooid cortices obtained by laser ablation inductively coupled plasma mass spectrometry show that these ooids have middle rare earth element enrichment prn smn 0 7 0 1 and smn ybn 1 5 0 4 and high fe contents 8200 1800 ppm this suggests ferruginous pore waters which were related to the selective or preferential absorption and release of rees by fe mn oxyhydroxides

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

controls of temperature alkalinity and calcium carbonate reactant on the evolution of dolomite and magnesite stoichiometry and dolomite cation ordering degree an experimental approach abstract natural and synthetic dolomites exhibit large scatter of structural and compositional characteristics with respect to variations in their stoichiometry and cation ordering degree cod but the physicochemical factors controlling dolomitization reactions are still poorly understood and by far not quantified in the present study dolomite was synthesized in batch reactors at temperatures of 150 c 180 c and 220 c by reacting calcite or aragonite with an artificial solution containing mgcl2 and nahco3 over a 360 day period the obtained results on the evolution of the stoichiometry of dolomite and magnesite and on the cod of dolomite indicate that the formation of stoichiometric and well ordered dolomite and of stoichiometric magnesite follow a ripening process and proceed through dissolution and re precipitation of a sequence of intermediate phases such as ca magnesite huntite very high mg calcite vhmc and disordered dolomite the initial occurrence and the metastability of the precursor phases were dependent particularly on reaction temperature and to a lesser extent on the caco3 source used temperature is the major rate limiting parameter of the evolution of dolomite and magnesite stoichiometry and dolomite cod corroborating results from prior experiments and natural deposits from our data we determined kinetic rates for the stoichiometric ripening of magnesite and dolomite and for the dolomite cod for the first time using a first order reaction kinetic approach the kinetics increase with temperature following the linear arrhenius equation in turn allowing for activation energies of the respective processes to be determined 44 0 57 4 and 9 3 kj mol 1 respectively interestingly we found the reaction rate for magnesite stoichiometric ripening is approximately one third of that for dolomite regardless of the temperature carbonate alkalinity or caco3 phase used for reaction furthermore we see that the activation energies required for the formation of ideal dolomite decreases in sequence from the precipitation of an initial vhmc phase 133 5 kj mol 1 arvidson and mackenzie 1999 followed by stoichiometric ripening 57 4 kj mol 1 to cation ordering 9 3 kj mol 1 for perfect ordering extrapolation of the obtained dolomitization rates indicates that about 1 4 and 6 8 myr are required to approach ideal dolomite at 50 and 25 c respectively explaining the large absence of ordered dolomite in modern sedimentary successions the present dataset provides new insights into dolomitization pathways and rates and contributes to a better understanding of the wide range of stoichiometries and cod values of dolomite and its evolution versus magnesite observed in the geological record and in laboratory studies

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

sr isotopic composition of paleoproterozoic 13c rich carbonate rocks the tulomozero formation se fennoscandian shield abstract rb sr systematics has been studied in 13c rich carbonate rocks of the paleoproterozoic 2 09 0 07 ga tulomozero formation in the northern onega lake area the se fennoscandian shield the formation is divided into eight members a f consisting of greenschist facies grade variegated sandstones siltstones mudstones stromatolitic dolostones and subordinate crystalline limestones samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the tulomozero formation prior to isotope analysis the rocks powders were treated with 1n ammonium acetate for partial removal of the late epigenetic carbonate phases major resetting of the rb sr systems in the tulomozero carbonate rocks appears to take place during the svecofennian regional metamorphic event and it was screened by using mn sr fe sr mg ca and 18o 16o ratios high sr content up to 2080 g g in limestones and 530 g g in dolostones coupled with low fe mn 0 40 in dolostones of the upper part of member b and of members g and h is indicative of seawater influxes sea transgression into the tulomozero basin the 87sr 86sr values in the least altered mn sr

energetics of calcium rich dolomite abstract the enthalpy of formation of sedimentary ca rich dolomite has been determined by oxide melt calorimetry the results show that ca rich dolomite is energetically different from well ordered stoichiometric dolomite the enthalpy of formation from calcite and magnesite becomes strongly more endothermic with increasing excess ca content this supports the idea that the substitution of ca in the mg layer of dolomite is energetically unfavorable ca rich dolomite is unstable relative to well ordered stoichiometric dolomite and calcite the enthalpy behavior for excess ca substitution in dolomite is different from that of mg substitution in calcite the enthalpy change is much larger in magnitude in dolomite and is more strongly dependent on composition differences in cation order as well as the presence of a modulated structure and low symmetry domains in ca rich dolomite cannot be discerned from the enthalpy data the findings confirm that the growth and occurrence of ca rich dolomite in sedimentary environments must be attributed to kinetic factors rather than to equilibrium

ca and mg isotope fractionation during the stoichiometric dissolution of dolomite at temperatures from 51 to 126 c and 5 bars co2 pressure natural polycrystalline hydrothermal sainte colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0 1 mol kg nacl solutions at 51 75 121 and 126 c and a pressure of 5 bars co2 in total 52 27 16 and 12 respectively of the dolomite placed in the reactors dissolved into the fluid phase during these experiments each experiment lasted from 12 to 47 days and the fluid phase in each evolved towards but did not exceed ordered dolomite equilibrium at a ph of 5 9 0 3 all aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite the reactive fluid compositions at the end of the experiments had a molar ca mg ratio equal to that of the dissolving dolomite and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy the ca and mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75 c in contrast the ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126 c was significantly greater such that 44 42casolid fluid 0 6 0 1 whereas that of mg was within uncertainty equal to that of the dissolving mineral the equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure our results at 121 and 126 c therefore are consistent with the one way transfer of mg from dolomite to the fluid but the two way transfer of ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution the lack of mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous mg as more than 12 of the dolomite dissolved during the 121 and 126 c closed system experiments our observations indicate a significant change in the ca isotopic signature of the dolomite during its stoichiometric dissolution moreover as there is no visual evidence for dolomite recrystallization during this experiment it seems likely that the resetting of ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems

identification of connate water in interstitial solution of chalk sediment interstitial solution was recovered from core samples of a borehole through 472 m of the upper cretaceous chalk formation in eastern england the solution has been analysed for major and some minor components and also for oxygen and hydrogen isotopic composition samples of the carbonate rock have also been analysed following acid dissolution the general feature of interstitial solution chemistry is a progressive dilution of dissolved species with decreasing depth from a near marine composition found in the deepest samples which are identified as connate water departures from the expected dilution trend are interpreted as evidence for diagenetic reactions during burial or as a result of freshwater circulation following subaerial uplift sr2 li and f have been enriched in interstitial solution and k has been slightly depleted the profile of chalk chemistry with depth shows no conclusive evidence for changes in the mg and sr content of the bulk carbonate due to freshwater diagenesis the 18o and d of the interstitial water reflect the same mixing as that shown by dissolved species however an apparent depletion of 18o and d in the deepest fluids with respect to the expected marine isotopic composition is tentatively explained as a result of the differences in diffusion rates of aqueous oxygen and hydrogen and of ionic solutes

dissolution methods for strontium isotope stratigraphy whole rock analysis the preparation of samples for strontium isotope stratigraphy sis by whole rock dissolution results in contributions of sr from contaminant phases as well as from the target carbonate phase here we highlight and quantify the errors in 87sr 86sr values that might result when calcium carbonate is dissolved using an inappropriate method we report the variation in 87sr 86sr value of sr released from four samples of differing calcite content and degrees of cementation when subject to different methods of dissolution we show that errors of up to 0 000076 in 87sr 86sr which correspond to errors of 3 68 ma in age can be avoided by careful preleaching of the samples

dolomites of the boat harbour formation in the northern peninsula western newfoundland canada implications for dolomitization history and porosity control the boat harbour formation constitutes the upper part of the lower ordovician st george group on the northern peninsula western newfoundland it ranges in thickness from 140 m 459 ft at main brook to 96 m 315 ft at daniel s harbour dolomitization of the carbonate sequence is more pervasive in the lower 30 to 40 m 98 to 131 ft at main brook whereas at daniel s harbour the section is entirely dolomitized petrography suggests that the boat harbour formation has been affected by three phases of dolomitization the earliest near surface or synsedimentary phase is d1 dolomicrite 4 55 m which exhibits dull to no luminescence it commonly occurs as laminae capping cycles and as breccias in the younger dolomite phases it has low sr 228 30 ppm and an average 18o value of 6 0 0 8 vienna peedee belemnite vpdb in the main brook section but more depleted signatures for 18o of 8 8 1 vpdb and lower sr contents 45 8 ppm in the daniel s harbour section the geochemical composition suggests that d1 was developed from fluids of a mixture of meteoric and marine waters the midburial phase d2 dolomite consists of coarse planar subeuhedral crystals 30 400 m that show concentric cathodoluminescence zoning and are also crosscut by microstylolites its 18o values range between 6 6 1 3 vpdb at main brook and 9 0 0 5 vpdb at daniel s harbour this dolomite likely precipitated from fluids that circulated through crustal rocks with progressive burial th value of 114c 11c and salinity value of 23 1 8 eq wt nacl the late stage d3 dolomite has large and coarse nonplanar crystals 125 m 7 mm that exhibit sweeping extinction under crossed polars which is characteristic of saddle dolomite and also sometimes shows thin brightly luminescent rims it was likely precipitated during deeper burial in pulses and from hot fluids th values of 148c 19c and 115c 19 6c and mean salinities of 23 2 and 22 2 eq wt nacl at main brook and daniel s harbour respectively this is also supported by their relatively more depleted 18o 11 1 1 2 and 12 3 1 4 vpdb respectively and low sr contents 88 36 and 38 5 9 ppm respectively porosity in the boat harbour formation is mainly associated with the midburial d2 dolomite intercrystalline porosity is the dominant type and it ranges in the formation from less than 1 to 8 at main brook and from 7 to 12 at daniel s harbour vugs are less common but are associated with d3 dolomite the porous zone in the formation at main brook starts approximately 10 to 15 m 33 to 49 ft below the lower boat harbour disconformity and extends down to the lower formational boundary whereas porous zones in the formation at daniel s harbour are indiscriminately distributed throughout the section

spatial variations in the stoichiometry and geochemistry of miocene dolomite from grand cayman implications for the origin of island dolostone abstract the cayman formation miocene 140 m thick on grand cayman is incompletely dolomitized with the most of the dolomite restricted to the peripheral part of the island these calcium rich dolomites with 50 60 mol caco 3 ca are divided into low calcium dolomite lcd ca 55 despite the small size of the island 6 8 km wide the percentages of lcd and hcd the ca and the geochemical properties of the dolomites show geographic variations relative to the surrounding shelf edge accordingly the cayman formation on the east end of the island is divided into the peripheral dolostone shelf edge 1 5 km inland transitional dolostone 1 5 2 7 km inland and the interior limestone and dolostone 2 7 km to island center from the peripheral dolostone to the interior limestone and dolostone there is an increase in the percentage of hcd and ca in the dolomite and decreases in the 18 o and 13 c values of the dolomite interpretations based on the oxygen and carbon isotopic compositions indicate that seawater is the source of mg for dolomitization the concentric pattern of dolomitization on the island reflects the fact that seawater flowed into the island from all directions during dolomitization the lateral inland variations in the dolomite stoichiometric and isotopic properties reflect the gradual modification of seawater by water rock interaction along the flow path from the shelf edge to the island center 87 sr 86 sr ratios indicate that two phases of dolomitization late miocene early pliocene and late pliocene early pleistocene were responsible for dolomitization of the cayman formation it is very likely that during both phases the carbonate platform was subaerially exposed and that the pump for circulating the seawater through the island was related to the seawater freshwater mixing zone the dolomitization model developed from grand cayman may be applicable to many other island dolostones affected by long term sea level changes given that the diagenetic potential of dolostone is directly linked to its stoichiometry the distribution of the lcd hcd will influence the petrographic properties geochemical signatures and reservoir potential of the dolostones

combined osmium and strontium isotopic study of the cretaceous tertiary boundary at sumbar turkmenistan a test for an impact vs a volcanic hypothesis osmium and strontium isotopic ratios at the cretaceous tertiary boundary at sumbar turkmenistan display a negative hyperbolic covariation superimposed on the long term trend which displays a positive covariation the minimum ratio for os and the maximum ratio for sr occur at the boundary clay volcanism with a mantle or crustal source cannot account for the isotopic data the low 187 o s 188 o s and the high 87 sr 86 sr ratios can be explained by an impact whereby os was derived from the bolide and the 87 sr 86 sr ratio was enhanced by acid rain and or a tsunami following the event