assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures though many studies on the mg contents in the calcitic tests of foraminifers exist the processes controlling its uptake are still a matter of debate laboratory cultures offer an excellent opportunity to reveal these mechanisms the mg concentrations within single chambers of the planktic foraminifer globigerinoides sacculifer brady maintained under controlled laboratory conditions were measured 1 at variable temperatures 19 5 29 5 c and constant salinity and 2 at variable salinity 22 45 and constant temperature the experimental results suggest that under natural conditions temperature is the leading mechanism controlling the mg ca ratio temperature and magnesium are related proportionally a temperature increase of ca 10 c gives rise to an increase of the magnesium concentrations of ca 130 drastic unnatural salinity changes dominate the effects of temperature a 110 change in the mg ca ratio was observed when salinity was elevated or reduced by more than ca 10 specimens which underwent gametogenesis reveal significantly higher mg concentrations than specimens that did not release gametes partition coefficients for mg in foraminiferal calcite are orders of magnitude lower than values from inorganically precipitated calcite when comparing observed mg ca ratios of foraminiferal tests with predicted mg ca ratios calculated according to empirical equations it becomes evident that foraminiferal tests are undersaturated with respect to mg for the water temperature they have experienced apparently foraminifers are capable of controlling their mg concentration the physiological processes presumably responsible for such depressed mg ca ratios appear to be temperature controlled as deduced from the close relationship of the observed mg ca ratios and water temperature this study demonstrates that variations in temperature and salinity are definitely reflected in the mg content of foraminiferal tests magnesium may thus serve as a paleo proxy for past surface water temperatures as long as postdepositional changes and salinity variations are of subordinate importance or can be excluded

reconstructing past seawater mg ca and sr ca from mid ocean ridge flank calcium carbonate veins proxies for past seawater chemistry such as mg ca and sr ca ratios provide a record of the dynamic exchanges of elements between the solid earth the atmosphere and the hydrosphere and the evolving influence of life we estimated past oceanic mg ca and sr ca ratios from suites of 1 6 to 170 million year old calcium carbonate veins that had precipitated from seawater derived fluids in ocean ridge flank basalts our data indicate that before the neogene oceanic mg ca and sr ca ratios were lower than in the modern ocean decreased ocean spreading since the cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early tertiary may explain the recent rise in these ratios

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

accuracy of stable mg and ca isotope data obtained by mc icp ms using the standard addition method the standard addition method is evaluated to verify the accuracy and precision of mg and ca isotope data with complex matrices using the standard sample bracketing technique and analysis by mc icp ms the 44ca 42ca ratio of seawater expressed as 44 42ca relative to srm915a was determined as 0 93 0 03 95 confidence in agreement with estimates obtained by the double spike method using standard addition the seawater 26mg 24mg ratio expressed as 26mg relative to the dsm3 standard was determined as 0 80 0 06 95 confidence in agreement with previous estimates four terrestrial silicate rocks morb flood basalt glacial flour and granodiorite and olivine mineral separates from an island basalt are shown to exhibit no scatter within the error of the method averaging a 26mg of 0 20 0 05 95 confidence although a number of silicate rock data for mg isotope ratios have already been reported this is the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after chemical purification of samples data regressions were evaluated statistically using the mean square weighted deviate mswd demonstrating that the uncertainty on individual data points are generally over estimated the external two standard deviation uncertainty on individual data points is estimated by monte carlo simulation as b0 075 about a factor of two improvement on early publications of mg isotope data the consistency of the standard addition estimates of 26mg in silicate rocks imply that if any residual matrix effects are present then they must be less than the spread of the data 0 11 given the diverse range of matrices in each of the samples the 26mg values of the silicate rocks suggest that mg isotope ratios in silicate material may only have a very restricted range the 26mg values of silicate material in the present study falls between the average values reported by teng et al teng f z wadhwa m helz r t 2007 investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle earth and planetary science letters 261 84 92 doi 10 1016 j epsl 2007 06 004 and wiechert and halliday wiechert u halliday a n 2006 non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 360 371 doi 10 1016 j epsl 2007 01 007 and given the spread of published 26mg values for chondritic material a chondritic composition for terrestrial mg cannot be ruled out we suggest that some of the small discrepancies between our data and analysis of the same samples in earlier studies may have arisen because the chemical purification of mg prior to analysis can easily induce analytical artifact this method could be expanded to the isotope ratios of other elements which also rely on correcting for mass bias using the standard sample bracketing method where similar analytical discrepancies may also exist

magnesium isotope fractionation in biogenic and abiogenic carbonates implications for paleoenvironmental proxies abstract geochemical variations in marine biogenic carbonates that are preserved in the geological record serve as proxies of past environmental change however interpreting most proxies is complicated by biologically mediated vital effects highlighting the need to develop new tools for reconstructing paleoenvironmental change recently magnesium mg isotope variability in carbonates has been explored extensively to determine its utility as a paleoenvironmental proxy we review the results of these works which have yielded valuable information on the factors affecting mg isotope fractionation between carbonates and solution 26mgcarb sol in biogenic and abiogenic carbonate minerals strong evidence exists for a mineralogical control on 26mgcarb sol with the negative offset from 0 following the sequence aragonite 3 mol mol and saturation states 3 that are similar to seawater suggest that 26mgcarb sol has a temperature dependence of 0 01 c 1 and is insensitive to precipitation rate in contrast a significant precipitation rate dependence is observed in calcites precipitated from solutions with relatively low mg ca ratios

secular variation in seawater chemistry during the phanerozoic as indicated by brine inclusions in halite analysis of data on the chemical composition of primary fluid inclusions brine inclusions in primary bedded halite from many evaporite formations of northern pangea shows that during the phanerozoic the chemical composition of marine brines was oscillating significantly between the na k mg ca cl type and the na k mg cl so4 type we regard those changes as corresponding to the chemical evolution of the phanerozoic ocean the changes correlate in time with earlier suggested secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites in addition to those secular changes of seawater chemistry the concentrations of k mg ca and so4 ions in marine brines underwent important changes under the influence of local factors including rock water interaction these secondary changes did not influence the principal chemical type of the brine characteristic for a given time interval in the past concentration of the ca ion did not exceed the present concentration in marine water by a

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 isotopic composition of magnesium in mantle olivine records of depletion and metasomatism we have investigated detailed mg isotopic variations in the lithospheric mantle by analysing olivine in mantle derived peridotite xenoliths and megacrysts high precision in situ analyses of mg 26 mg 24 and mg 25 m 24 g are made using a laser ablation microprobe and mc icpms measurements are done using a standard sample bracketing technique with an in house olivine standard replicate analyses of this standard give a precision of 0 20 parts per thousand 2sd for delta mg 26 mg 26 mg 24 sample mg 26 mg 24 standard 1 x 1000 and 0 12 parts per thousand 2sd for delta mg 25 the analysed olivine grains represent the lithospheric mantle beneath archean cratons siberia kaapvaal slave and phanerozoic fold belts se australia results from olivines show significant heterogeneity in the lithospheric mantle delta mg 26 ranges from 3 01 parts per thousand to 1 03 parts per thousand and delta mg 25 from 1 59 parts per thousand to 0 51 parts per thousand relative to the magnesium isotopic standard dsm 3 there is a broad trend from lighter mg isotopic compositions in depleted xenoliths from archean mantle to heavier mg in the less depleted phanerozoic samples samples with petrographic evidence of refertilisation including modal metasomatism show large ranges in delta mg values within samples sheared peridotite xenoliths from the kaapvaal and slave cratons show a shift to higher delta mg associated with the introduction of fluids with an asthenospheric signature olivine in the least metasomatised peridotites from se australia has isotopically light mg whereas olivine in cryptically and modally metasomatised peridotites amphibole apatite bearing becomes progressively heavier both absolutely and relative to pyroxene and amphibole with increasing degrees of metasomatism the heterogeneity measured in individual samples suggests that mg isotopic fractionations produced by processes of mantle metasomatism are preserved on the intra grain scale and the magnitude of the observed fractionations indicates that diffusion related kinetic processes are important in controlling isotope fractionation at high temperatures the in situ measurement of mg isotopes provides a powerful new method for investigating processes in the mantle c 2005 elsevier b v all rights reserved

mg isotope fractionation during calcite precipitation an experimental study abstract experimental precipitations of calcite and other carbonate minerals were performed under various conditions of ph temperature and solution mg ca to determine the mg partition coefficient and mg isotope fractionation fifteen experiments were performed at ph ranging from 7 41 0 07 to 8 51 0 39 temperature ranging from 16 2 0 7 to 26 5 0 3 c and mg ca solution ranging from 0 11 to 0 52 mol mol the apparent mg partition coefficient between calcite and solution d mg spans a large range of values from 0 018 0 014 to 0 15 0 11 and carbonate mg isotope fractionation 26 mg ranges from 2 53 0 25 to 1 33 0 14 and does not correlate with either ph or temperature the range in d mg and 26 mg suggests non equilibrium partitioning controlled by the processes of calcite growth i e mixing between calcite grown at equilibrium and fluid inclusions and entrapment of a surface mg rich calcite layer in isotopic equilibrium with the solution the equilibrium mg isotope fractionation between inorganic calcite and solution is estimated to be 2 13 0 24 additional mg elemental and isotopic fractionations are observed to occur during biogenic formation of calcite due to variable removal of mg by the organisms high mg calcite corals foraminifera of seawater mg from their calcification medium

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

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

the behaviour of magnesium and its isotopes during glacial weathering in an ancient shield terrain in west greenland the magnesium mg isotope composition of rivers is sensitive to changes in the balance of primary mineral dissolution and secondary mineral formation as these processes are regulated by climate then changes to the earth s climate system such as the onset of glaciation could potentially alter the isotopic behaviour of mg in rivers to investigate how mg isotopes behave during glacial weathering we have determined the mg concentration and mg isotope ratio for the dissolved suspended and bedload phases of glacial and non glacial rivers in west greenland this region is essentially monolithological and there is little biological activity hence isotope variations largely result from differences in weathering processes dissolved mg in rivers is always enriched in the light mg isotopes relative to the bedload 26mg 0 4 and the glacial rivers have lighter mg isotope compositions 26mg 1 to 1 3 than the non glacial rivers 26mg 0 6 enrichment of light mg in the dissolved load is consistent with the preferential uptake of heavy mg from solution during the formation of secondary silicate minerals however evidence from saturation state modelling suggests that little secondary mineral formation is likely to have occurred in the glacial rivers because the concentration of dissolved solids is very low moreover the glacial derived suspended sediment has identical mg isotope ratios to the bedload these observations suggest that the formation of secondary weathering minerals is not controlling the behaviour of mg isotopes in these rivers rather variations in the 26mg value of the dissolved load are attributed to incongruent weathering of the solid phase preferential dissolution of carbonate minerals which have low 26mg 1 1 to 5 2 imparts a more negative 26mg signal to the glacial river waters although our study indicates that the mg isotopic composition of the dissolved load is sensitive to changes in weathering congruence it is unlikely that glaciation will result in major changes in the 26mg composition of continental runoff

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

the isotopic composition of magnesium in the inner solar system abstract we report high precision mc icpms measurements of 25 mg 24 mg and 26 mg 24 mg isotope ratios for 14 meteorites both primitive and differentiated 5 lunar basalts and breccia samples 5 terrestrial basalts 1 dunite and 3 peridotites we estimate the 26 mg value of the bulk silicate earth bse at 0 54 0 04 2se w r t dsm3 carbonaceous and ordinary chondrites show an average 26 mg of 0 52 0 04 2se martian meteorites show a narrow range in 26 mg with an average 26 mg of 0 57 0 02 2se average 26 mg of pallasite olivines is 0 54 0 04 2se whereas that of lunar samples including ilmenite basalts olivine basalt and one crystalline breccia is 0 51 0 03 2se our data demonstrate that the overall stable mg isotopic composition of the inner solar system average 25 mg 0 273 26 mg 0 535 is homogeneous to within 0 04 for 26 mg 24 mg this value is indistinguishable from that of the bse and lunar samples demonstrating that the mg isotopic composition of the earth moon system is chondritic accurate determination of the mg isotopic composition of the rocky planets is important for mg isotope studies of surface processes on earth as well as for using mg isotope data to understand evaporation and condensation effects in the solar nebula consistent with a recent report we find that clinopyroxenes have higher 26 mg than co existing olivines and orthopyroxenes in mantle peridotites although the magnitude of this difference varies 0 08 to 0 2 such small mineral fractionations along with differences between components in primitive meteorites cais chondrules and matrix preclude the resolution of mass dependent fractionation differences smaller than about 0 05 between solar system objects we demonstrate that our results do not support the recent claims that i the earth moon system has a unique mg isotope composition different from chondrites or ii the higher 26 mg values up to 0 0 reported for chondrites achondrites terrestrial basalts and peridotites to address the issue of inter laboratory discrepancies on the mg isotopic composition of silicate rocks we have prepared 3 pure mg standards with wide ranging 26 mg 3 75 to 0 69 with respect to dsm3 as well as a large volume of homogenized olivine from the san carlos peridotite which will be available to the scientific community for future comparative studies

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

lithium magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands this study presents major and trace elements and uranium u lithium li and magnesium mg isotope data for dissolved and suspended particulate material from estuaries draining dominantly basaltic terrains in iceland borgarfjor ur and sao miguel povoacao in the azores archipelago the concentrations of dissolved sodium na calcium ca potassium k sulphate so4 mg and li and 7li 6li and 26mg 24mg isotope ratios vary directly as a function of the dissolved chloride concentration cl indicating that these species are non reactive in these estuaries however u appears to be affected either by colloid flocculation or the formation of fe oxyhydroxides in borgarfjor ur the major element composition of suspended particulate material in the estuary at borgarfjor ur shows that the detrital silicates delivered to the estuarine mixing zone are relatively unweathered accordingly the 26mg isotope composition of the suspended material is indistinguishable from that of the parent basalts however the 234u 238u activity ratios of the suspended material are significantly different from the parent basalt indicating rapid equilibration of the u isotopes between particles and seawater the li concentration and 7li isotope composition of the suspended load are also observed to increase with increasing cl consistent with ongoing weathering of detrital material and the formation of secondary minerals in seawater depending on the particle residence time in the estuary this process could remove as much as 15 25 of the global riverine input of li to the ocean

on the secular variations in the composition of phanerozoic marine potash evaporites in a recent paper l a hardie proposed that the secular changes in the mineralogy of marine nonskeletal limestones and in the mineralogy of marine potash evaporites during the phanerozoic are the result of changes in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid ocean ridges he suggested that even quite small variations 25 in this flux have a profound effect on the composition of seawater we show that the effects of changes in the hydrothermal flux are much smaller than he proposed nevertheless a doubling of the hydrothermal flux could probably alter the composition of seawater sufficiently to affect the mineralogy of marine potash evaporites an alternative explanation for the variations in the mineralogy of these deposits invokes differences in the degree of dolomitization during the evaporation of seawater the apparent near constancy of the k sup content of seawater during the phanerozoic supports this hypothesis it seems likely that during periods of rapid seafloor spreading the rate of seawater cycling is greater than today that sea level is higher that very large carbonate platforms are more abundant and that changes in seawater chemistry together with increases in dolomitization during the evaporation of seawatermore lead to mgso sub 4 free potash deposits the relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several phanerozoic periods 27 refs 1 fig 1 tab less

the source of ions for biomineralization in foraminifera and their implications for paleoceanographic proxies the global carbon cycle is strongly perturbed by fossil fuel burning leading to atmospheric co2 increase climatic warming followed by polar ice melting and global sea level rise are predicted due to the greenhouse effect of increasing co2 in the atmosphere houghton et al 1995 the ocean plays a major role in neutralizing the excess co2 because the amount of inorganic carbon available for exchange with the atmosphere in the ocean is approximately 50 60 times larger than in the atmosphere e g siegenthaler and sarmiento 1993 the bulk of the atmospheric co2 excess will eventually be neutralized by caco3 dissolution in the deep marine environment this is however is a relatively slow process that operates on the time scale of ocean circulation 1000 yrs and is therefore causing an accumulation of co2 in the atmosphere this phenomenon will result in potentially severe consequences to the well being of global ecological systems obviously the scientific attention of many biogeochemists is focused on processes controlling the response of the marine system to changes in atmospheric co2 concentrations e g archer and maier reimer 1994 broecker 1997 sigman and boyle 2000 berger 2002 foraminifera corals and coccolithophores in the global carbon cycle there are four dominant processes involved in neutralizing the excess atmospheric co2 in the ocean these are 1 gas exchange at the air sea interface and reaction with the carbonate ion to form bicarbonate 2 net primary productivity 3 caco3 production and dissolution and 4 ocean circulation most of these processes are biologically mediated and may have special importance in shallow tropical environments where the exchange with the atmosphere is more direct in this review we address the mechanism of biomineralization in one of the major groups that precipitates caco3 in the ocean the foraminifera as we will show this group

mg isotopic systematics during early diagenetic aragonite calcite transition insights from the key largo limestone abstract the seawater magnesium mg isotopic composition 26mgsw can be applied to quantify marine mg cycle in earth s history however reconstruction of 26mgsw by using marine carbonate has been challenged by the large range of variation in mg isotopic composition of carbonate 26mgcarb such variation on the one hand reflects e g the mineralogy temperature and precipitation rate dependent isotopic fractionation during ca carbonate precipitation on the other hand the pristine seawater record could be modified in carbonate diagenesis during which less stable carbonate minerals would spontaneously convert to more stable mineral phases in the modern ocean with high mg ca molar ratio 5 aragonite precipitation is favored however aragonite is thermodynamically less stable than low mg calcite at ambient surface environment resulting in the spontaneous transition from aragonite to low mg calcite in the earliest stage of diagenesis i e the aragonite calcite transition as a common process in the modern ocean mg isotopic fractionation in aragonite calcite transition may provide valuable constraints on the variation of 26mgcarb in this study we measured mg isotopic compositions of pleistocene key largo limestone florida which has been experiencing the aragonite calcite transition during early diagenesis the key largo limestone samples consist of a mixture of calcite and aragonite 26mg of aragonite 26mgarag shows higher and more stable values while calcite has lower yet more variable mg isotopes 26mgcal we suggest a model conceptualizing a water film attached to the mineral surface in which the aragonite calcite transition initiates with aragonite dissolution and the mg exchange with external fluid can be variable followed by mg isotopes fractionate during low mg calcite precipitation in this model 26mgcal is controlled by 1 the chemical composition of water film which is dependent on the mineralogical composition of aragonite i e mg content and 26mgarag the composition of external fluid i e fresh water seawater or mixed fresh water and seawater and the mixing ratio between dissolved aragonite and external fluid and 2 the isotopic fractionation during calcite precipitation because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution therefore 26mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes further complicating the interpretation of carbonate data thus the process controlled mg isotopic fractionation should be taken into consideration when reconstructing 26mgsw by using carbonate sediments rocks

influences of temperature and mg ca ratio on caco3 precipitates from seawater the mg ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature results indicate a dramatic change in the critical mg ca ratio over a relatively small temperature range for example in normal seawater with a mg ca of 5 1 the critical temperature is 6 3 c at a temperature of 25 c however mg ca must be less than 1 4 0 1 for calcite to form these data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors such as paleo p co 2 p pressure and paleo mg ca ratios that may control carbonate mineral formation by abiotic processes however the inverse relation between inferred global paleoclimatic conditions and the dominant type of caco 3 polymorph being abiotically deposited i e warm conditions and low mg calcite versus cool conditions and aragonite suggest that substantial changes must have taken place in ocean chemistry over phanerozoic time

brine inclusions in halite and the origin of the middle devonian prairie evaporites of western canada abstract brines were extracted from fluid inclusions in lower salt halite of the middle devonian prairie formation in saskatchewan canada the brines were analyzed by ion chromatography and were found to be of the na k mg ca cl type they do not fall along a simple evaporation trend brines from clear diagenetic halite are significantly lower in na and higher in mg2 ca2 and cl than brines from cloudy subaqueously formed halite with chevron structures the isotopic composition of strontium and sulfur in anhydrite associated with the halites was found to be the same as that of middle devonian seawater the composition of the inclusion brines can be derived from that of modern seawater by evaporation extensive dolomitization of limestone a d albitization of clay minerals other evolution paths are however also feasible and it is impossible to rule out effects due to the addition of nonmarine waters hydrothermal solutions surface runoff and groundwater or dissolutional recycling of existing evaporites within the prairie evaporite basin our analyses and published data on brine inclusions in halite from a number of phanerozoic evaporite deposits show that the na k mg ca cl type brine is more common than the na k mg cl so4 type which is expected from evaporation of modern seawater

stable isotopic and elemental characteristics of recent tufa from a karstic krka river south east slovenia useful environmental proxies tufa samples from 16 consecutive barrages along a 13 km section of a groundwater fed krka river slovenia were analysed for their petrographical mineralogical elemental and stable carbon 13c and oxygen 18o isotope composition to establish their relation to current climatic and hydrological conditions waters constantly oversaturated with calcite and the steep morphology of the krka riverbed stimulate rapid co2 degassing and subsequent tufa precipitation the carbon isotope fractionation 13c between dic and tufa in the krka river evolves toward isotopic equilibrium being controlled by continuous co2 degassing and tufa precipitation rate downstream the 13c increased from 1 9 to 2 5 vienna peedee belemnite however since tufa precipitation rates remain similar downstream the major controlling factor of carbon isotope exchange is most probably related to the continuous 12co2 degassing downstream leaving the carbon pool enriched in 13c in the case of oxygen the isotope fractionation was found to be from 1 0 to 2 3 smaller than reported in the literature the observed discrepancies are due to different precipitation rates of calcite deposits as krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools due to non equilibrium oxygen isotope exchange between krka tufa and water the 18o proxy showed from 1 2 to 8 2 c higher calculated water temperatures compared to measured water temperatures demonstrating that 18o proxy based temperature equations are not reliable for water temperature calculations of fast growing tufa on cascades because mg is bound to the terrigenous dolomite fraction in the krka tufa samples the mg ca was also found to be an unreliable temperature proxy yielding over up to 20 c higher calculated water temperatures this article is protected by copyright all rights reserved

model of seawater composition for the phanerozoic we present an inverse model of phanerozoic seawater com position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites the model considers step wise alteration of seawater composition via 1 variable input of river water 2 variable rates of alteration of seawater through reactions at mid ocean ridges and 3 variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general the model achieves agreement with paleoseawater fluid inclusion data for na 1 c a 2 1 s o 4 2 2 and k 1 particularly when variable runoff is considered variable rates of basalt seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater particularly the observed near constant concentration of k 1 through time

time series δ26mg analysis in speleothem calcite kinetic versus equilibrium fractionation comparison with other proxies and implications for palaeoclimate research abstract magnesium isotope time series mc icp ms analyses from a pleistocene speleothem collected in a limestone cave in nw africa morocco are reported and discussed in a process oriented context in addition high resolution c o and sr isotope data and mg and sr element abundances were compiled from the same stalagmite sub samples were collected along the stalagmite growth axis and a second data set was drilled perpendicularly within one growth interval hendy test the analytical results show clearly co variant systematic and cyclical fluctuations for all proxies collected along the growth axis and with respect to the analytical error invariant data within one growth increment magnesium isotope ratios 26 mg fluctuate between 4 39 0 02 2 and 4 17 0 05 2 and are within the range of published results of speleothem calcite from limestone caves the difference of 0 22 is significantly beyond the error of the external reproducibility of 0 03 2 for 26 mg considering the analytical error neither a purely kinetic nor an equilibrium fractionation process explains the observed isotope pattern for the time being it is suggested that two external factors drive the speleothem mg isotope cyclicity 1 climate driven arid versus humid variances in the precipitation rate of a carbonate phase from meteoric water within the karstic system prior to entering the cave system and 2 changing rates of silicate aeolian material versus carbonate weathering both of these processes fractionate the mg isotopic composition of runoff seepage water there is evidence that the magnesium isotope system applied to speleothem archives bears significant information concerning continental climatic variability in arid zones and deserves further research

towards a better understanding of magnesium isotope ratios from marine skeletal carbonates abstract this study presents magnesium stable isotope compositions of various biogenic carbonates of several marine calcifying organisms and an algae species seawater samples collected from the western dutch wadden sea and reference materials the aim of this study is to explore the influence of mineralogy taxonomy and environmental factors e g seawater isotopic composition temperature salinity on magnesium isotopic 26mg ratios of skeletal carbonates using high precision multi collector inductively coupled plasma mass spectrometry we observed that the magnesium isotopic composition of seawater from the semi enclosed dutch wadden sea is identical to that of open marine seawater we further found that a considerable component of the observed variability in 26mg values of marine skeletal carbonates can be attributed to differences in mineralogy furthermore magnesium isotope fractionation is species dependent with all skeletal carbonates being isotopically lighter than seawater while 26mg values of skeletal aragonite and high magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences the 26mg values of echinoids brachiopods and bivalves likely result from a taxon specific level of control on mg isotope incorporation during biocalcification moreover no resolvable salinity and temperature effect were observed for coralline red algae and echinoids in contrast mg isotope data of bivalves yield ambiguous results which require further validation the data presented here point to a limited use of mg isotopes as temperature proxy but highlight the method s potential as tracer of seawater chemistry through earth s history

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

impact of biomineralization processes on the mg content of foraminiferal shells a biological perspective the mg ca ratio in foraminiferal shells is widely used as a proxy for paleotemperatures nevertheless it seems that the basic mg content of foraminifera is determined by biological factors as can be concluded from the large inter species and intrashell variability and the frequent deviations from inorganic behavior this paper discusses three possible ways by which foraminifera can control or modify the mg content in their shell 1 involvement of organic matrix in the precipitation process that may alter the partition coefficient of mg in biogenic calcite 2 controlled conversion of transient amorphous phases to calcite and 3 modification of the mg concentration in the parent solution from which the crystals precipitate the first two mechanisms are probably responsible for the precipitation of high mg calcite phases whole shell or sublayers while the third mechanism leads to the formation of low mg calcite phases we propose a model adapted from epithelial cells that allows massive mg2 removal from the biomineralization site this model is especially relevant to the planktonic and deep benthic low mg foraminifera that are frequently used for paleotemperature reconstructions we discuss the possible biological roles of mg in the shell in terms of the calcite polymorph conservation the in vivo chemical stability of the shell the functions of mg as a stabilizer of transient phases and as a controlling agent of the precipitation process several temperature sensitive biological processes that may influence the mg ca ratio of the shell are suggested and a model that combines biogenic and inorganic considerations is presented the model uses mg heterogeneity in the shell together with temperature response biologic and inorganic of biomineralization processes to account for the deviation of planktonic foraminifera from inorganic calcite at equilibrium with seawater

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

tracking physicochemical conditions of evaporite deposition by stable magnesium isotopes a case study of late permian langbeinites magnesium isotopic compositions of evaporite deposits may record information concerning brine evolution during deposition we report mg isotopic values delta mg 26 dsm3 measured from an evaporite deposit of langbeinite k2mg2 so4 3 found in the permian salado formation we used these data to model mg isotope fractionation between langbeinite and its parent brine in addition both measured and theoretical results are used to estimate precipitation temperature and interpret depositional environment the salado langbeinite delta mg 26 values are relatively low and fall within a relatively narrow range 4 12 0 03 parts per thousand to 3 81 0 07 parts per thousand equilibrium fractionation factors between langbeinite and aqueous mg2 solutions were calculated using quantum chemical density functional theory all computations were performed at the b3lyp 6 31 g d p level solvation effects were addressed using a solvent model water droplet approach and mineral structures were investigated using volume variable cluster models vvcm the equilibrium mg isotopic fractionation factors a between langbeinite and model brine solution we obtained are 1 0005 1 0004 and 1 0003 delta mg 26 langb water approximate to 10 3 ln alpha 0 473 parts per thousand 0 390 parts per thousand and 0 322 parts per thousand at 10 degrees c 25 degrees c and 40 degrees c respectively these relatively large equilibrium fractionation factors indicate significant mg isotope fractionation between langbeinite and its parent brine during precipitation as langbeinite preferentially incorporates the heavier mg 26 and mg 25 isotopes rayleigh distillation modeling of the salado langbeinite s relatively light mg isotopic composition requires delta mg 26 dsm3 values of 4 parts per thousand for the parent brine models favor a precipitation temperature as high as 40 degrees c under equilibrium conditions potential disequilibrium precipitation conditions suggested by mg isotopic data also imply rapid deposition in a hot arid sedimentary environment prevailing in the southwestern u s during the late permian

relocated hypocenters and structural analysis from waveform modeling of aftershocks from the 2011 prague oklahoma earthquake sequence abstract we present an analysis of aftershocks with m l 3 from the 2011 prague oklahoma earthquake sequence an intraplate sequence of moderate size earthquakes within the north american craton we apply waveform analysis to seismograms from temporary local seismograph networks to relocate the aftershocks hypocenters the relocations show that the hypocenters are confined to the precambrian basement the character of the recorded seismograms waveforms are reconstructed by finite difference forward modeling of seismic sources buried at different depths in 2d crustal models the numerical modeling indicates that the observed complex waveforms and particularly the coda are created by waves scattering off numerous shallow crustal velocity anomalies we infer that these anomalies correspond to paleoriver channels that are embedded in the pennsylvanian limestone formation the modeling indicates that the amount of energy within the coda depends on the depth of the source our waveform modeling of the observed seismograms is consistent with our relocations of the aftershocks to depths of 5 10 km within the precambrian basement electronic supplement empirical and synthetic data model parameters and geological figures

precise magnesium isotope measurements in core top planktic and benthic foraminifera 1 this study presents a new methodology to obtain highly precise measurements 0 1 of magnesium isotope ratios in very small samples of foraminiferal carbonate 40 50 g here this technique is used to examine mg isotopic variation among different species of core top foraminifera over a range of different ambient conditions despite the high degree of temperature control on the abundance of elemental mg incorporated into foraminiferal tests analyses of surface dwelling globigerinoides ruber and g sacculifer from five cores with associated sea surface temperatures ranging from 20 to 31 c show no significant temperature dependent variations in their mg isotope ratios analyses of different size fractions of g sacculifer show an increase in mg ca with test size but no variation of mg isotope ratio in all nine planktic and benthic species were analyzed all show identical mg isotope ratios with a mean of 26mg 4 72 apart from small differences in three species namely o universa g sacculifer which are both 0 4 lighter than the average and p obliquiloculata which is 0 4 heavier these results highlight the constancy of foraminiferal mg isotope ratios despite changing environmental conditions which dominate mg ca variation and arguably affect ca isotope fractionation this is an important observation which needs to be included in any model of foraminiferal calcification the insusceptibility of 26mg values to external parameters makes mg isotopes ideally suited to constraining past variations in the mg isotope budget of the oceans and the information this carries about the history of oceanic dolomitization continental weathering and hydrothermal behavior

what do we know about the evolution of mg to ca ratios in seawater 1 although most reconstructions of the evolution of the mg to ca ratio in seawater conclude that it has increased during the course of the cenozoic they disagree widely regarding the magnitude of this change on the basis of fluid inclusion and caco3 mineralogy observations the increase was at least threefold on the basis of mg content of foraminifera shells it was only a factor of 1 7 a recently published reconstruction based on the mg content of calcite fillings of voids in ridge flank basalts lends support to the conclusion that the change was severalfold but as it is very difficult to come up with a plausible geologic scenario which could account for such a large change we lean toward the smaller estimate based on the magnesium content of foraminifera shells

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

modeling novel stable isotope ratios in the weathering zone when rock is converted to weathering products the involved processes can be fingerprinted using the stable isotope ratios of metals for example li mg ca fe sr and metalloids b si here we construct a framework for interpreting these novel stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context the approach is applicable to any novel stable isotope system and is based on a simple steady state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays and uptake of nutrients by plants the model results show that the isotope composition of a given element in the weathering zone compartments depends on 1 the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material consisting of secondary precipitates and organic matter 2 the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants a relationship is established between isotope ratios isotope fractionation factors and indexes for chemical weathering such as chemical depletion fractions cdf and elemental mass transfer coefficients derived from simple elemental concentration measurements from this relationship isotope fractionation factors can be calibrated from chemical and isotope data measured on field material furthermore we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock water and sediment this calculation can be applied to samples from soils from rivers and from the sedimentary record and does not require knowing the isotope fractionation factors involved in the reactions finally we apply the model to the oceanic li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the cenozoic

mg isotopic composition of carbonate insight from speleothem formation simultaneous high precision measurement of 24 mg 25 mg and 26 mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry mc icp ms relative to the international standard srm980 data are presented on low mg calcite speleothems and their associated host rocks and waters from four caves one in the french alps and three in israel covering various climate conditions in addition data are presented on three dolostones and three limestones from the himalaya the overall variation is 4 13x and 2 14x in n 26 mg and n 25 mg respectively this is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of mg is not unique each speleothem shows a characteristic range of n 26 mg values that are attributed to the isotopic composition of the local water differences between the isotopic composition of mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to mg isotopic fractionation during carbonate precipitation in the temperature range of 4 18 c the low mg calcite is enriched in light isotopes by 1 35x amu and the dependence on temperature has been found to be less than 0 02x amu c despite various geological settings the n 26 mg of the studied dolostones is 2 0 1 2x higher than the n 26 mg of the limestones all together these results suggest a strong mineralogical control and a weak temperature effect on the mg isotopic composition of carbonate a 2002 elsevier science b v all rights reserved

influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater a constant disequilibrium technique was used to determine the composition of magnesian calcite overgrowths precipitated on pure calcite seeds from artificial seawater at 5 25 and 40 c the amount of magnesium incorporated in the overgrowths at a given temperature is independent of the precipitation rate over a wide range of saturation states and is believed to correspond to a composition in true equilibrium with seawater the distribution coefficient of magnesium dmg2 c in the magnesian calcite overgrowths increases almost linearly with temperature being 0 0121 0 0013 at 5 c 0 0172 0 0022 at 25 c and 0 0271 0 0013 at 40 c these values apply only to magnesian calcites precipitated from standard composition seawater since a previous study has shown dmg2 c to be a function of the mg2 ca2 ratio in the parent solution results of this study are compared with values reported previously by other workers and with the compositional distribution of naturally occurring magnesian calcite cements and ooids found in seawater it appears that variations in temperature are not sufficient to account for the compositional variability of naturally occurring inorganic marine magnesian calcite cements

magnesium isotopic fractionation in chondrules from the murchison and murray cm2 carbonaceous chondrites we present high precision measurements of the mg isotopic compositions of a suite of types i and ii chondrules separated from the murchison and murray cm2 carbonaceous chondrites these chondrules are olivine and pyroxene rich and have low 27 al 24 mg ratios 0 012 0 316 the mg isotopic compositions of murray chondrules are on average lighter d 26 mg ranging from 0 95 to 0 15 relative to the dsm 3 standard than those of murchison d 26 mg ranging from 1 27 to 0 77 taken together the cm2 chondrules exhibit a narrower range of mg isotopic compositions than those from cv and cb chondrites studied previously the least altered cm2 chondrules are on average lighter average d 26 mg 0 39 0 30 2se than the moderately to heavily altered cm2 chondrules average d 26 mg 0 11 0 21 2se the compositions of cm2 chondrules are consistent with isotopic fractionation toward heavy mg being associated with the formation of secondary silicate phases on the cm2 parent body but were also probably affected by volatilization and recondensation processes involved in their original formation the low al cm2 chondrules analyzed here do not exhibit any mass independent variations in 26 mg from the decay of 26 al with the exception of two chondrules that show only small variations just outside of the analytical error in the case of the chondrule with the highest al mg ratio a type iab chondrule from murchison the lack of resolvable 26 mg excess suggests that it either formed 1 ma after calcium aluminum rich inclusions or that its al mg isotope systematics were reset by secondary alteration processes on the cm2 chondrite parent body after the decay of 26 al

the effect of growth rate on uranium partitioning between individual calcite crystals and fluid abstract elemental to calcium ratios in calcium carbonate minerals are used to study environmental conditions in particular uranium to calcium ratio u ca has been proposed as a proxy for seawater carbonate ion concentration co 3 2 and seawater ph this work is focused on the evaluation of growth rate and its effect on uranium partitioning between calcite and fluid we grew inorganic calcite by diffusion of co 2 isothermally from nh 4 cl cacl 2 doped with uranium the growth rate of calcite crystal extension rate v was monitored by sequentially spiking calcite precipitating fluids with rare earth element ree dopants the ree was analyzed with secondary ion mass spectrometry sims at spots matching those where u ca was determined partition coefficient k u u ca calcite u ca fluid increases with increasing growth rate v specifically k u increases from 0 02 to 0 06 when v increases from 0 01 to 0 14 nm s and remains nearly constant at faster rates numerical simulations using the growth entrapment model gem and unified uptake kinetics model uukm were undertaken to explain k u v relationship in the recent data on calcite

application of calcite mg partitioning functions to the reconstruction of paleocean mg ca abstract calcite mg ca is usually assumed to vary linearly with solution mg ca that a constant partition coefficient describes the relationship between these two ratios numerous published empirical datasets suggests that this relationship is better described by a power function we provide a compilation of these literature data for biotic and abiotic calcite in the form of calcite mg ca f solution mg ca h where f and h are empirically determined fitting parameters describing the slope and deviation from linearity respectively of the function this is equivalent to freundlich sorption behavior controlling mg incorporation in calcite using a power function instead of a partition coefficient lowers phanerozoic seawater mg ca estimates based on echinoderm skeletal material by on average 0 5 mol mol from previous estimates these functions can also be used to model the primary skeletal calcite mg ca of numerous calcite phases through geologic time such modeling suggests that the mg ca of all calcite precipitated from seawater has varied through the phanerozoic in response to changing seawater mg ca and that the overall range in mg ca measured among various calcite phases would be greatest when seawater mg ca was also high e g aragonite seas and lowest when seawater mg ca was low e g calcite seas it follows that during times of calcite seas when the seawater mg ca is presumed to have been lower deposition of calcite with low mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and in turn lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites