Mass-Independent Oxygen Isotope Fractionation and Its Application to Earth Sciences
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Mass-independent oxygen isotope fractionation provides new insights to the research on global changes.Based on an introduction to mass-independent isotope fractionation,this paper discusses the definition of oxygen isotope anomaly [Δ(17O)] and the production mechanisms of massindependent oxygen isotope fractionation,particularly the application of massindependent oxygen isotope fractionation to earth sciences.The productivity assessed with Δ(17O) is total biosphere productivity.It removes the limitation of only evaluating terrestrial or oceanic productivity individually and establishes a basis for the productivity estimates in a more broad temporal and spatial scale.In particular,using Δ(17O) to quantify effectively the relative contribution of homogenous and heterogeneous reaction pathways of aerosol sulfate and nitrate opens a new way for investigating the interaction between climate and aerosol.The combination of Δ(17O) and S isotope in the ice core not only traces the source and transport of sulfate and nitrate but also provides detailed information on their oxidation processes.The discovery of sulfate and nitrate Δ(17O) in some arid areas can reasonably reduce the great uncertainty of identifying the sources and genesis of some sediments.This demonstrates that mass-independent oxygen isotope fractionation will play a more important role in the research on(ancient) atmospheric ozone activity,chemistry in volcanic plumes and O,S and N biogeochemical cycle.Keywords:
Equilibrium fractionation
Biogeochemical Cycle
Mass-independent fractionation
Sulfate aerosol
Ice core
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Stability of the isotopic composition of carbon in endogenic terrestrial substances, as well as in meteorites, consistently lower in C13 than the biogenic marine carbonates, suggests both presence and stability of a certain zone under the crust of the earth in which the C systems are maintained at certain equilibria, at different levels, typical of certain geological processes operative therein. Isotopic exchanges and recurrent fractionations of the C isotopes, in the course of the migrations of carbon, are indicated by the available evidence, the net result of which is an impoverishment of C13 in living substance and in its derivatives (oils, coals, etc.), and its enrichment in the biogenic residual carbonates. – IGR Staff.
Carbon fibers
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Recent studies targeting the metabolic, physiological, and biochemical controls of sulfur isotope fractionation in microbial systems have drawn linkages between results from culture experiments and the sulfur isotope signatures observed in natural environments. Several of those studies have used newer techniques to explore the minor isotope (33S and 36S) variability in those systems, and also have attempted to place them in an ecophysiological context. Sparingly few have incorporated this newfound understanding of minor isotope behavior into natural systems (sediment pore waters, water columns) and none of them have refined existing isotope-dependent reaction-transport models to explicitly include 33S. In this study, we construct a three-isotope (32S, 33S, and 34S) reaction-transport model of pore water sulfate for a well-characterized sedimentary system within the California-Mexico Margin (Alfonso Basin). An additional goal is placing recent laboratory culture work into a natural, physical context. The model first reproduces the measured bulk geochemical characteristics of the pore water profiles of [SO42−], [CH4], dissolved inorganic carbon ([DIC]), and [Ca2+]—and predicts bulk (non-isotope-specific but depth-dependent) rates of sulfate reduction. Next, the model uses those depth-dependent bulk rates, in combination with empirically calibrated fractionation factors, to explain the minor isotope characteristics (δ34S and Δ33S values) of the 0 to 40 cm pore water SO42−. The down core, isotopic evolution of pore water sulfate requires a large fractionation associated with sulfate reduction (34εSR = 70 ± 5‰) that appears to be independent of bulk rate, but in line with low temperature thermodynamic predictions. The minor isotope characteristics (33λSR ∼ 0.5130) are also independent of rate and fall within the range expected from microbial calibrations, but differ from minor isotope predictions of thermodynamic equilibrium. The high value of 34εSR raises key questions in relating the physiological state of marine microorganisms relative to their laboratory counterparts, as well as point toward exceedingly low metabolic rates in natural marine sediments.
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Comparison of experimental data with analyses of oceanic sulfate indicates that oceanic sulfate is not in oxygen isotope equilibrium with ocean water. Preliminary experiments suggest that the turnover of sulfate in the sulfur cycle is too rapid to allow equilibrium to be established. If this is so, the sulfur cycle must exert a significant influence on the oxygen balance of the oceanatmosphere system.
Sulfur Cycle
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Stable isotopes of atmospheric carbon dioxide (CO 2 ) contain a wealth of information regarding biosphere‐atmosphere interactions. The carbon isotope ratio of CO 2 (δ 13 C) reflects the terrestrial carbon cycle including processes of photosynthesis, respiration, and decomposition. The oxygen isotope ratio (δ 18 O) reflects terrestrial carbon and water coupling due to CO 2 ‐H 2 O oxygen exchange. Isotopic CO 2 measurements, in combination with ecosystem‐isotopic exchange models, allow for the quantification of patterns and mechanisms regulating terrestrial carbon and water cycles, as well as for hypothesis development, data interpretation, and forecasting. Isotopic measurements and models have evolved significantly over the past two decades, resulting in organizations that promote model‐measurement networks, e.g., the U.S. National Science Foundation's Biosphere‐Atmosphere Stable Isotope Network, the European Stable Isotopes in Biosphere‐Atmosphere Exchange Network, and the U.S. National Environmental Observatory Network.
Carbon fibers
Terrestrial ecosystem
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Natural abundances of stable isotopes of nitrogen and carbon (${\delta}^{15}N$ and ${\delta}^{13}C$ ) are being widely used to study N and C cycle processes in plant and soil systems. Variations in ${\delta}^{15}N$ of the soil and the plant reflect the potentially variable isotope signature of the external N sources and the isotope fractionation during the N cycle process. $N_2$ fixation and N fertilizer supply the nitrogen, whose ${\delta}^{15}N$ is close to 0%o, whereas the compost as. an organic input generally provides the nitrogen enriched in $^{15}N$ compared to the atmospheric $N_2$ . The isotope fractionation during the N cycle process decreases the ${\delta}^{15}N$ of the substrate and increases the ${\delta}^{15}N$ of the product. N transformations such as N mineralization, nitrification, denitrification, assimilation, and the $NH_3$ volatilization have a specific isotope fractionation factor (${\alpha}$ ) for each N process. Variation in the ${\delta}^{13}C$ of plants reflects the photosynthetic type of plant, which affects the isotope fractionation during photosynthesis. The ${\delta}^{13}C$ of C3 plant is significantly lower than, whereas the ${\delta}^{13}C$ of C4 plant is similar to that of the atmospheric $CO_2$ . Variation in the isotope fractionation of carbon and nitrogen can be observed under different environmental conditions. The effect of environmental factors on the stomatal conductance and the carboxylation rate affects the carbon isotope fractionation during photosynthesis. Changes in the environmental factors such as temperature and salt concentration affect the nitrogen isotope fractionation during the N cycle processes; however, the mechanism of variation in the nitrogen isotope fractionation has not been studied as much as that in the carbon isotope fractionation. Isotope fractionation factors of carbon and nitrogen could be the integrated factors for interpreting the effects of the environmental factors on plants and soils.
Isotopes of nitrogen
Nitrogen Cycle
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The unique and distinctive 17O-excess (Δ17O) of ozone (O3) serves as a valuable tracer for oxidative processes in both modern and ancient atmospheres. This isotopic signature is propagated throughout the atmospheric reactive nitrogen (NOx = NO + NO2) cycle and preserved in nitrate (NO3-) aerosols and mineral deposits, providing a conservative tracer for the relative importance of ozone and other key oxidant involved in NOx cycling. However, despite the intense research effort dedicated to the interpretation of Δ17O(NO3-) measurements, the atmospheric processes responsible for the transfer of Δ17O to nitrate and their overall influence on nitrate isotopic composition on different spatial and temporal scales are not well understood. Furthermore, due to the inherent complexity of extracting ozone from ambient air, the absolute magnitude and spatiotemporal variability of Δ17O(O3) remains poorly constrained, a problem that has confounded the interpretation of Δ17O measurements for over a decade. The research questions that have been pursued in this thesis were formulated to address these knowledge gaps. The primary analytical tool used was the bacterial denitrifier method followed by continuous-flow isotope ratio mass spectrometry (CF-IRMS), which allows for the comprehensive isotopic analysis of nitrate (i.e., δ15N, δ18O, Δ17O). This method was applied to the isotopic analysis of nitrate samples in two case studies: (i) an investigation of the diurnal and spatial features of atmospheric nitrate isotopic composition in coastal California; and (ii) a study of the seasonality and air-snow transfer of nitrate stable isotopes on the Antarctic plateau. Furthermore, the method was adapted to the isotopic characterization of ozone via chemical conversion of its terminal oxygen atoms to nitrate. During the course of this thesis, a large dataset of tropospheric Δ17O(O3) measurements has been obtained, including a full annual record from Grenoble, France (45°N) and a ship-based latitudinal profile from 50°S to 50°N in the Atlantic marine boundary layer (MBL). This observational dataset represents a two-fold increase in the number of existing tropospheric Δ17O(O3) observations and a dramatic expansion in the global representation of this key isotopic variable. Additionally, the two case studies presented reveal novel and often unexpected aspects of the isotope dynamics of atmospheric nitrate, with potentially important implications for air quality modeling and the interpretation of isotopic information preserved in the polar ice core record.
TRACER
Isotopic signature
Isotope Analysis
δ18O
Reactive nitrogen
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Analyses of stable and radioactive isotope compositions have become a mainstay of the chemical perspective of oceanography, owing in large part to their value as tracers of important oceanographic processes. The utility of isotopes as tracers of biological, physical and geological ocean processes is perhaps the main reason that chemical oceanography has become a strongly interdisciplinary science. Small contrasts in stable isotope compositions can carry geographic information for discriminating sources such as different ocean water masses, and marine versus terrestrially derived organic matter. Within fossils, isotope distributions afford information about the temperatures, geographic settings, transport mechanisms, and ecology (e.g. who ate whom) of ancient environments. Stable isotope compositions also integrate the cumulative results of ongoing processes such as the passage of organic elements up trophic levels, climate change, marine productivity, and the formation and melting of continental glaciers. Stable isotopic signatures can persist over geologic time, even through severe changes in chemical composition. Radioactive isotopes have the additional property of being useful as nuclear clocks that, regardless of environmental conditions, dependably tick away to indicate the age of an object or the dynamics (e.g. turnover time) of a pool of materials. In addition, nuclear decay events often involve conversions of parent isotopes to daughter elements with very different physical and chemical properties, which then can be sensitively traced as they seek new chemical forms and locations in the ocean.
Isotope Geochemistry
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Calcium isotope,one of non -traditional stable isotopes,has been a research hotspot in isotope geochemistry field since the last decade.Calcium isotope ratios are widely measured by TIMS or MC-ICP-MS,and are expressed as δ44/40Ca or δ44/42Ca respectively.Variations of δ44/40Ca in nature are mainly from -2.0‰ to 2.0‰,spanning only a limited range of 4.0‰ .The hypotheses of kinetic and equilibrium fractionation can interpret some of calcium isotope fractionation observations,but more work is needed to better understand the fractionation mechanism.Up to now the geological application of calcium isotope includes:(1)Paleoceanography temperature reconstruction based on δ44/40Ca of planktonic foraminifera G.sacculifer;(2) Geochemical cycling of calcium in ocean;(3)Estimation of pCO2 in terms of seawater calcium concentration.
Isotopes of calcium
Isotope Geochemistry
Equilibrium fractionation
Isotopes of strontium
paleoceanography
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Biogeochemical Cycle
Biogeochemistry
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