稳定同位素分馏蒸汽压效应的计算方法

稳定同位素分馏的蒸汽压效应（vaporpressureisotopeeffects,简称VPIE）,在地球化学和天体化学上有着非常重要的研究意义。大部分情况下,由于轻重同位素体具有不同的蒸汽压,在经历挥发和蒸发过程时,含有重同位素的物种挥发得慢,轻同位素物种挥发得快,最终结果导致凝聚相富集重同位素,气相含有较多的轻同位素。在地球化学上,VPIE直接同非常重要的地学参数——同位素平衡分馏系数仅联系在一起。本文应用Bigeleisen提出的方法,直接将VPIE和约化配分函数比（RPFR）相联系,只需要通过理论计算获得两种物质的简谐振动频率,就能够得到非高压情况下该物质的VPIE。本文以水和硫镉矿（CdS）为例,详细介绍了如何计算蒸发和气化过程VPIE的方法,并指出了其在天体化学和矿床学中的一些潜在应用。     

Chondritic Mg isotope composition of the Earth

The processes of planetary accretion and differentiation have potentially been recorded as variations in the stable isotope ratios of the major elements between planetary objects. However, the magnitude of observed isotopic variations for several elements (Mg, Fe, Si) is at the limit of what current analytical precision and accuracy are able to resolve. Here, we present a comprehensive data set of Mg isotope ratios measured in ocean island and mid-ocean ridge basalts, peridotites and chondrites. The precision and accuracy were verified by isotopic standard addition for two samples, one carbonaceous chondrite (Murchison) and one continental flood basalt (BCR-1). In contrast with some previous studies, our data from terrestrial and chondritic materials have invariant Mg isotope ratios within the uncertainty of the method (0.1‰ for the ²⁶Mg/²⁴Mg ratio, 2SD). Although isotopic variations of less than about 0.1‰ could still be present, the data demonstrate that, at this level of uncertainty, the bulk silicate Earth and chondritic Mg reservoir have a homogeneous δ²⁶Mg = −0.23‰ (²⁶Mg/²⁴Mg ratio of the sample relative to the DSM3 standard set to zero by definition). This implies that neither planetary accretion processes nor partial mantle melting and subsequent shallow-level differentiation have fractionated Mg isotope ratios. These observations imply in particular that the formation of the Earth cannot stem from preferential sorting of chondrite constituents that would have been fractionated in their Mg isotope composition. It also implies that unlike oxygen isotopes, there was no zonation in Mg isotopes in the inner solar system.     

Nuclear field shift effects on stable isotope fractionation: a review

An anomalous isotope effect exists in many heavy element isotope systems (e.g., Sr, Gd, Zn, U). This effect used to be called the “odd–even isotope effect” because the odd mass number isotopes behave differently from the even mass number isotopes. This mass-independent isotope fractionation driving force, which originates from the difference in the ground-state electronic energies caused by differences in nuclear size and shape, is currently denoted as the nuclear field shift effect (NFSE). It is found that the NFSE can drive isotope fractionation of some heavy elements (e.g., Hg, Tl, U) to an astonishing degree, far more than the magnitude caused by the conventional mass-dependent effect (MDE). For light elements, the MDE is the dominant factor in isotope fractionation, while the NFSE is neglectable. Furthermore, the MDE and the NFSE both decrease as temperatures increase, though at different rates. The MDE decreases rapidly with a factor of 1/T², while the NFSE decreases slowly with a factor of 1/T. As a result, even at high temperatures, the NFSE is still significant for many heavy element isotope systems. In this review paper, we begin with an introduction of the basic concept of the NSFE, including its history and recent progress, and follow with the potential implications of the inclusion of the NFSE into the kinetic isotope fractionation effect (KIE) and heavy isotope geochronology.     

Lithium and boron isotopes in illite-smectite: The importance of crystal size

Clay minerals record chemical data about the past, acting like natural computer memory chips. To retrieve the data we must understand how they are stored. To achieve this we have examined the isotopic information revealed by two trace elements, lithium and boron, that are incorporated into the common clay minerals illite-smectite (I-S) during diagenesis. We used hydrothermal experiments at 300°C, 100 MPa, to speed up the reaction of smectite to illite that normally occurs during slow (10-100 Ma) sediment burial. During illitization, Li substitutes into the octahedral sites and B enters the tetrahedral sites of the silicate framework. Both Li and B are also adsorbed in the interlayer of smectite, but Li is preferred over B in the exchange sites. To determine the equilibrium isotope fractionation of the two trace elements it is important to remove these adsorbed interlayer species. By measuring the isotopic composition of Li and B in the silicate framework during reaction, we can address the relative timing of element exchange in the different crystallographic sites. Furthermore, because illitization of smectite is a crystal growth process (not an isomorphous replacement) we have examined the effect of crystal size on the isotope fractionation.The results show that Li and B approach an isotopic steady state when R1 ordering occurs, long before oxygen isotopes equilibrate with the fluid. The isotopic fractionation (α_{mineral-water}) for Li (0.989) is similar to that for B (0.984) at 300°C. However, when separated into <0.2, 0.2-2.0, and >2.0 μm fractions, there are significant differences in measured isotope ratios by as much as 9‰. Crystal growth mechanisms and surface energy effects of nanoscale crystals may explain the observed isotopic differences. The fact that different crystals equilibrate at different rates (based on size) may be applied to natural samples to reveal the changing paleofluid history, provided we understand the conditions of equilibrium. This has very important implications for the interpretation of diagenetic environments, fluid flow, and surficial geochemical cycling.     

钛同位素地球化学综述

随着分析技术的进步,非传统稳定同位素体系在地球化学、天体化学和生物地球化学等研究领域的应用日益广泛。钛(Ti)是一个非常重要的过渡族金属元素,在地球和其他类地球行星中广泛存在。但是由于Ti是一种难熔的、流体不活动性元素,高温地质过程中Ti同位素分馏很小。人们对Ti同位素体系的地球化学应用的关注相对其他非传统稳定同位非常有限。而近年来,随着化学纯化方案的优化以及双稀释剂方法的改进和仪器质谱性能的提高,Ti同位素组成的高精度测试已经能够实现。天然样品中Ti同位素组成的变化随之得以发现,使得学者们能够利用这一新的稳定同位素体系来解决与高温和低温地球化学相关的问题。很快Ti同位素体系地球化学研究成为当前国际地质学界的前沿研究课题和新的发展方向之一。本文首先在简要介绍Ti元素和Ti同位素体地球化学性质的基础上,介绍了Ti元素化学分离和Ti同位素分析方法。随后笔者总结了已有的不同类型球粒陨石和地球样品的质量相关Ti同位素组成研究结果,对硅酸盐地球的Ti同位素组成做了初步评估。前人对高温地质样品的Ti同位素组成研究初步探明Ti同位素在岩浆演化过程,例如部分熔融和结晶分异等重要地质过程中的分馏行为。笔者在此基础上探讨了结晶分异过程中引起Ti同位素分馏的主要控制因素,指出Ti同位素是潜在的研究岩浆演化过程的新工具。最后笔者探讨了Ti同位素地球化学未来的发展方向,以加速我国在Ti同位素地球化学方面的应用研究。     

Study of exchangeable metal on colloidal humic acids and particulate matter by coupling ultrafiltration and isotopic tracers: Application to natural waters

A new method is proposed to characterize the complexation properties of colloids and metal–colloid interactions in natural waters. Based on the association of ultrafiltration with isotopic tracing, this method could quantify the pool of elements in an exchangeable position and also address the kinetic aspects of these exchanges. Basically, it consists of the comparison of isotopic compositions between the bulk sample and a succession of filtrates through time. Exchanges between colloidal humic acids (HA) and metals were first characterized, before applying such manipulations on natural waters. A few elements, representative of a wide range of complexation properties, were chosen: Cu, Zn, Cd, Pb, Sr, Nd, Ni, Th and U. In the case of humic acids, very small (less than 10% of isotopes), but significant isotopic shifts were observed compared to the isotopic equilibrium. It means that more than 90% of the isotopes were exchanged just after addition of isotopic tracers. Experiments on natural organic-rich waters (Mengong and Nyong streams) indicate isotopic composition variations close to those of humic acids. On the contrary, ultrafiltration performed on the total Sanaga River water (including suspended matter “SM”) shows an important isotopic shift between the filtered and unfiltered solutions. It means that in the case of the Sanaga River, a significant part of the chemical elements did not exchange.     

Application of the stable-isotope system to the study of sources and fate of Hg in the environment: A review

With the improvement of analytical methods and the development of multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP/MS), research on non-traditional stable isotope (Cu, Zn, Fe, Se, Mo, Cr, Hg) in geochemistry has made tremendous progress in the past decade. Recent studies have demonstrated that both organic and inorganic reactions may cause Hg isotope fractionation, and variations of Hg isotopic composition in the environment have been successfully employed to explain Hg pollution history, Hg sources and tracking Hg pathways in nature. Furthermore, Hg isotopic fractionation studies can be a powerful tool in the calibration of global Hg cycling models. Stable isotope geochemistry of Hg is therefore becoming a new frontier subject in earth sciences. Based on summarizing previous research, this paper outlines the main advances in the study of Hg stable isotopes with particular emphasis placed on a brief explanation of Hg isotope analytical techniques, possible Hg isotope fractionation mechanisms observed in both natural and experimental processes, Hg isotope composition variations in different environmental matrices, and the application prospects of the Hg stable isotopes in environmental geosciences.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

SHRIMP in situ isotopic analyses of REE,Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (?-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.     

岩石样品中低含量铂族元素和锇同位素比值的高精度测量方法

铂族元素（Os,Ir,Pt,Ru,Rh,Pd)具有强亲铁性和强亲铜性，为一组地球化学性质相近的相容元素，铂族元素包含两个同位素衰变体系（^190Pt-^186Os和^187Re-^187Os)。近年来，铂族元素和Re-Os同位素在研究各类不同地持作用过程中，尤其是在地幔岩石的研究中，作用独特，效果显著。由于地幔岩石的铂族元素含量较低，因此高精度，高灵敏度的分析测试方法的研究就显得十分重要。以往的分析方法（如常规的ICP－MS和中子活化分析方法），对含10^-9-10^012级低含量铂族元素的产品分析精度一般较差（＞15％－100％）。所采用的分析流程通常也无法同时获得样品的铂族元素含量和Os同位素比值。本文采用新的熔样方法（HAP－S高温高压釜酸溶法），新的化学流程（溶剂萃取和阴离子交换树脂柱）和新的分析仪器（多接收等离子体质谱MC－ICPMS和负离子热电离质谱N－TIMS）。用同位素稀释法对低含量地幔橄榄岩样品同时测定的铂族元素含量和Os同位素比值，获得了高精度的分析结果。对所分析的地橄榄样品中的铂族元素分配曲线和Os同位素组成的地质意义进行了初步探讨。     

Mathematical simulation of the carbon isotopic fractionation between huminitic coals and related methane

In order to estimate the isotope fractionation effect between coals and methane during coalification a maturity-related fractionation model has been developed for coals and reservoir gases of NW Germany which is based on empirical data. Assuming that observed isotope shifts of the convertible carbon of coals of different maturities are related to a loss of methane during coalification and that this shift can be described by a Rayleigh distillation process, functions with preselected fractionation factors were fitted to measured isotope data of the convertible carbon of coals. The best approximation of theoretical and measured data was achieved with a low fractionation factor (α_c= 1.003). Using this model theoretical methane carbon isotope data were determined and compared to the isotopic composition of reservoir methanes of NW Germany. Although the methane isotope data of reservoir gases and the related maturity of the coals show a slight scatter, the theoretical data plot within the same range and follow the increase of the ¹³C concentration of reservoir gases with increasing maturity of the coals.     

The significance of isotope specific diffusion coefficients for reaction-transport models of sulfate reduction in marine sediments

Modeling isotopic signatures in systems affected by diffusion, advection, and a reaction which modifies the isotopic abundance of a given species, is a discipline in its infancy. Traditionally, much emphasis has been placed on kinetic isotope effects during biochemical reactions, while isotope effects caused by isotope specific diffusion coefficients have been neglected. A recent study by Donahue et al. (2008) suggested that transport related isotope effects may be of similar magnitude as microbially mediated isotope effects. Although it was later shown that the assumed differences in the isotope specific diffusion coefficients were probably overstated by one or two orders of magnitude (Bourg, 2008), this study raises several important issues: (1) Is it possible to directly calculate isotopic enrichment factors from measured concentration data without modeling the respective system? (2) Do changes in porosity and advection velocity modulate the influence of isotope specific diffusion coefficients on the fractionation factor α? (3) If one has no a priori knowledge whether diffusion coefficients are isotope specific or not, what is the nature and magnitude of the error introduced by either assumption? Here we argue (A) That the direct substitution of measured data into a differential equation is problematic and cannot be used as a replacement for a reaction-transport model; (B) That the transport related fractionation scales linearly with the difference between the respective diffusion coefficients of a given isotope system, but depends in a complex non-linear way on the interplay between advection velocity, and downcore changes of temperature and porosity. Last but not least, we argue that the influence of isotope specific diffusion coefficients on microbially mediated sulfate reduction in typical marine sediments is considerably smaller than the error associated with the determination of the fractionation factor.     

蒸发岩非传统稳定同位素研究综述

蒸发岩是海水/卤水蒸发浓缩的产物,不同的水化学环境下析出的蒸发岩类型不同,其在不同的地质历史时期都有分布,是重要的古海水和古环境记录载体。蒸发岩研究的主要问题包括:蒸发岩的物质来源、形成时代、蒸发盆地和卤水的演化历史、蒸发岩矿物所记录的环境变化。一些矿物、元素及同位素指标可用于解决这些问题,其中稳定同位素对于示踪蒸发岩的物质来源与形成过程具有不可替代的作用。近20年来,非传统稳定同位素地球化学获得快速发展,并在蒸发岩研究中获得成功应用,这些同位素体系包括阴离子元素B、Cl和Br,以及阳离子元素Mg、K和Ca。本文综述了多个非传统稳定同位素在蒸发岩领域的研究,主要包括:蒸发岩矿物与溶液之间的同位素分馏系数、蒸发岩的同位素信息重建古海水同位素组成及示踪蒸发岩成因和时代等。     

Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in ⁵⁶Fe/⁵⁴Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the ⁵⁶Fe/⁵⁴Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ⁵⁶Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ⁵⁶Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ⁵⁶Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ⁵⁶Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ⁵⁶Fe values; these seem best explained through production of high ⁵⁶Fe/⁵⁴Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ⁵⁶Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ⁵⁶Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.     

Major,trace element and Sr isotopic composition of lavas from Vico volcano (Central Italy) and their evolution in an open system

Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.     

Continuous 60-year stable isotopic and earth-alkali element records in a modern laminated tufa (Jaruga,river Krka,Croatia): Implications for climate reconstruction

A continuous 60-year record (1938–1998) of stable isotope compositions of carbon and oxygen, as well as trace metal (Mg, Sr, Ba) concentrations in a laminated calcite crust precipitated in a short artificial tunnel on a non-equilibrium groundwater-fed karstic river is presented. Chemical and isotopic records have been compared to hydrometeorological data, available for the last 48 years. An attempt is made to relate isotopic and geochemical variations in the crust to environmental parameters, such as temperature, precipitation and changes in vegetation cover, as well as to postdepositional recrystallisation of the older crust material. Isotopic composition of the crust is largely influenced by non-equilibrium precipitation, which favours the incorporation of isotopically depleted C and O into the carbonate. Furthermore, because of the complicated hydrological situation, there is no observable correlation between the stable oxygen isotope composition of water and temperature. The result is that the ¹⁸O isotopic thermometers overestimate the measured precipitation temperatures. Temperatures calculated from Mg/Ca ratios of water and the carbonate match the δ¹⁸O palaeotemperatures within ± 2.4 °C in the older part of the crust, precipitated before the onset of industrial pollution of the river. It was demonstrated that the application of Mg palaeothermometry in natural systems, where the Mg/Ca ratio of water is influenced not only by temperature, but also by other environmental parameters such as precipitation, surface runoff, groundwater retention time and anthropogenic influences, is subject to a large uncertainty, up to 10 °C.     

Fe-Cu-Zn同位素在矿床学和找矿勘查中的应用

金属稳定同位素已经广泛应用于矿床学研究以及找矿勘查。金属来源及其成矿过程是金属矿床研究以及找矿勘查中重点关注的基本核心问题。金属稳定同位素这项新技术进一步提高了我们对地壳中金属来源、迁移和富集的认识。这项新技术的优点是我们可以直接从矿石矿物本身获取信息。在本文中,我们重点关注矿石、水、岩石、土壤、植物等测量出的Fe-Cu-Zn同位素分馏,聚焦于Fe-Cu-Zn同位素从最深部岩浆系统开始一直向上延伸到浅部表生系统的过程中Fe-Cu-Zn同位素如何应用于矿床学研究以及找矿勘查,试图展示这些相对较新的技术可以提供的潜在应用范围。经过系统研究和总结,我们认为金属稳定同位素数据可以从三方面加以利用。首先,地表的植物、水、风化的岩石以及土壤中产生的较大的同位素分馏可以作为地下矿产勘查的指示标志;第二,矿区范围内金属稳定同位素往往具有系统的空间变化规律,可以指示成矿热液空间演化模式和矿体延伸方向;第三,金属元素作为成矿元素,其同位素可以直接有效地约束矿石的形成过程、成因以及源区特征。     

Intramolecular carbon isotopic analysis of acetic acid by direct injection of aqueous solution

We report an improved method for determining the intramolecular carbon isotopic composition of acetate using direct injection of aqueous samples. The system builds upon prior work that established pyrolytic conditions for online analysis and represents a significant advance in that it requires minimal preparation for samples containing as little as 1 mM sodium acetate in aqueous solution. The technique is applicable for analysis of oilfield brines, culture samples, biological samples and natural porewaters. We demonstrate its accuracy by use of a stable isotope dilution series. We also show that addition of a base and cryogenic preconcentration may induce an isotopic effect on the carboxyl carbon. This isotopic fractionation does not appear to extend to the measured methyl carbon isotope value although it can significantly alter the measured isotopic composition of the whole molecule. Our preconcentration experiments demonstrate that the method is suitable for carbon isotopic measurements of acetate methyl carbon in natural samples at concentrations as low as 90 μM, considerably broadening potential applications.     

Diagenesis and the reconstruction of paleoenvironments: A method to restore original δO values of carbonate and phosphate from fossil tooth enamel

Intra-tooth δ¹⁸O variations within the carbonate (δ¹⁸Oc) and phosphate (δ¹⁸Op) components of tooth apatite were measured for Miocene and Pliocene hypsodont mammals from Afghanistan, Greece and Chad in order to evaluate the resistance of enamel to diagenetic alteration. Application of water-apatite interaction models suggest that the different kinetic behaviours of the phosphate-water and carbonate-water systems can be used to detect subtle oxygen isotope disequilibria in fossil enamel when intra-individual variations are considered. Selective alteration of the oxygen isotope composition from the carbonate component of Afghan and Greek enamels suggests inorganic isotopic exchange processes. Microbially-induced isotopic exchange for phosphate is demonstrated for the first time in enamel samples from Chad, in association with extensive recrystallization. In Chad, δ¹⁸Op values were derived from partial isotopic exchange with fossil groundwater during early diagenesis. Mass balance calculations using average carbonate content in enamel as a proxy for recrystallization, and the lowest δ¹⁸Op value of dentine as a proxy for the isotopic composition of the diagenetic fluid, indicate that diagenesis can alter δ¹⁸Op by as much as 3‰ in some enamel samples. This diagenetic alteration is also responsible for a decrease in intra-individual variations of up to 1‰ in affected specimens. The effects of diagenesis on δ¹⁸Op values of fossil enamel are not systematic, however, and can only be estimated if sequential δ¹⁸Op and δ¹⁸Oc analyses are performed on fossil enamel and dentine. Reconstruction of large temporal- or spatial-scale paleoclimates based on δ¹⁸Op analyses from mammalian teeth cannot be considered valid if enamel has been affected by bacterial activity or if the data cannot be corrected for diagenetic effects.     

Possibilities and limitations of isotopic chemostratigraphy

Postsedimentary transformations of rocks impede the use of isotopic methods for the paleoecological reconstruction and correlation of geological events. This fact was demonstrated previously based on the study of Neoproterozoic-Cambrian rocks (Vinogradov, 2008). Detailed analysis of literature material shows that the younger (Paleozoic) rocks were also subjected to alterations of composition and initial isotope ratios. As compared to Precambrian rocks, Paleozoic rocks are characterized by a much narrower scatter in isotope ratios of cyclical elements. This situation significantly complicates the interpretation of isotopic variations, and correlation of geological events is difficult or even impossible. Thus, isotopic methods can apparently be used adequately for the chemostratigraphic analysis only since Mesozoic.