Apparent Fractionation Factor Research Articles

Oxygen isotope partitioning between phosphate and carbonate in mammalian apatite

The oxygen isotope compositions of phosphate and structural carbonate in mammalian enamel and bone apatite are linked to that of body water at constant body temperature near 37°C, but the isotope systematics of oxygen in structural carbonate are not well understood. Using coupled measurements of the oxygen isotope composition of structural carbonate and phosphate from horse tooth enamel, the apparent oxygen isotope fractionation factor between structural carbonate and body water is estimated to be 1.0263 ± 0.0014. These estimates provide a quantitative basis for using the oxygen isotope composition of structural carbonate in mammalian biogenic apatite for ecological, climatological, and physiological reconstruction.

Difference in diffusion coefficients of 14NO3 −-N and 15NO3 −-N affects the apparent fractionation factor associated with denitrification in soil

Abstract We previously analyzed the effect of nitrate dispersion on the apparent nitrogen isotope fractionation factor associated with denitrification in soil (Kawanishi et al. 1993), and found that the dispersion effect was significant when the water flow was slow. In the previous report, we assumed that the dispersion coefficients of 14NO3 − and 15NO3 − were similar. However, when the water flow is slow, molecular diffusion will dominate mechanical dispersion and the above assumption may not be valid.

Dispersion effect on the apparent nitrogen isotope fractionation factor associated with denitrification in soil; Evaluation by a mathematical model

An analytical model is constructed to investigate the effect of dispersion on the apparent 15N/ 14N fractionation factor associated with denitrification in soil. The steady input of solute with a fixed isotope ratio, uniform flow of the solute, and the first-order reaction for each isotope are assumed, and the relationship between the concentration distribution and the change of isotope ratio in the steady state is examined. The only dimensionless parameter, k 14N D/ v 2, the product of the first-order reaction rate constant and the dispersion coefficient divided by the square of the superficial velocity, determines the effect of dispersion, and if its value is larger than 0.01, the dispersion will affect the apparent isotope fractionation factor. As k 14N D/ v 2 increases, the effect becomes more prominent, and when it approaches infinity, the ratio of the apparent per mille enrichment factor to the true one, (α ap −1)/(α tr −1), reaches 0.5.

溶質輸送と脱窒とが同時に起こる有限のシステムによる見かけの同位体分別係数について

We constructed a numerical model that simulates a finite system where solute transport and denitrification and isotope fractionation associated with denitrification simultaneously occur. We described the system simply as one dimensional convective-dispersive equation with reaction in finite length, solved the equations numerically by FDM, and examined the effect of dispersion on the change of solute concentration and isotope ratio through the system.As Peclet number decreases and as the unreacted fraction of nitrate decreases, the relationship between the unreacted fraction and delta-15 value of effluent diverts from the relationship without the effect of solute transport, and the apparent fractionation factor diverts from the true one. The ratio of apparent to true (per mill) enrichment factors decreases as dimensionless reaction rate constant κ14N increases and Peclet number Pe decreases, not the ratio κ14N/Pe but both κ14N and Pe affect the ratio of apparent to true (per mill) enrichment factors, in this point, the apparent fractionation factor calculated by this finite system differs from that calculated from solute and isotope ratio distributions in the semi-infinite system.

Boron isotopic compositions of brine, sediments, and source water in Da Qaidam Lake, Qinghai, China

The isotopic compositions of boron in Da Qaidam Lake, Quinghai, have been determined by using the technique of thermal ionization mass spectrometry of cesium tetraborate. The brines are enriched in 11B, with an average δ 11B of +5.49 ± 2.34%., compared with those of lake sediments and boron minerals produced by the lake. The apparent isotopic fractionation factors between brine and lake sediments as well as boron minerals are 0.988 ± 0.001 and 0.986 ± 0.001, respectively. Source water δ 11B values range from −11.72 to + 16.69%., depending on their origin.

Lithium isotopic composition of submarine basalts: implications for the lithium cycle in the oceans

We have measured the Li isotope composition of young, pristine basalts from active ocean ridge crests, of progressively older basalts along a dredging transect, and a limited number of hydrothermally altered basalts. The data significantly extend our limited knowledge of the isotopic abundance ratio of Li in geological material. Fresh mid-ocean ridge basalts have δ 6Li values of between −3.4 and −4.7‰ relative to isotope standard L-SVEC. During low-temperature weathering on the seafloor, the isotopic composition of the rock becomes increasingly heavier due to addition of seawater Li ( δ 6Li= −32.3‰ ). The oldest (46 m.y.) and most altered rock studied has an isotopic composition of −14‰. A linear relationship exists between δ 6Li and the inverse of Li concentration, suggesting that Li in weathered basalts can be regarded as a two-component mixture of basaltic Li and seawater-derived Li that has been incorporated in alteration minerals, most likely secondary clays such as smectite and phillipsite. The inferred Li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1.019 relative to the seawater source. Thus Li uptake by secondary minerals from the low-temperature weathering process and, by analogy, incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter Li isotope from ocean water. However, isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources, unless the hydrothermal flux is comparable to the river flux. Alternatively, more important sinks of 6Li must exist if the steady state isotopic composition of Li in ocean water is to be maintained.

Oxygen isotope effects associated with the solid-state α-FeOOH to α-Fe2O3 phase transformation

Samples of synthetic and natural goethites were subjected to isothermal dehydration under both closed- and open-systein conditions at various teinperatures ranging from 160 to 300°C. The oxygen isotope ratios of the dehydration product (hematite) were systematically different for open- and closedsystein dehydration. In every instance, open-systein dehydration resulted in 18 O enrichments of the product hematite relative to the starting goethite. For all but a few cases, closed-system dehydration produced an 18 O depletion in the residual mineral relative to the starting goethite. These oxygen isotope effects indicate that significant mineral-vapor isotope exchange occurred during the solid-state, closed-system transformation of goethite to ∗∗ hematite. Calculated values of the apparent closed-system oxygen isotope fractionation factor between hematite and the corresponding liquid water suggest that equilibrium isotope exchange may have been approached in those samples for which the extent of closed-system dehydration was at least 95%. The rapidity of the mineral-vapor isotope exchange associated with the solid-state goethite to hematite transformation is indicated by the fact that the dehydration times (for those samples apparently approaching equilibrium) ranged from only 24 to 68 hours. These results suggest that the oxygen isotope ratios of many hematites in low temperature geological systems reflect the environmental conditions associated with their formation from some hydrous precursor. Information about the 18 O 16 O ratio in the hydrous precursor itself is not likely to be directly preserved in the hematite

Methane oxidation in sediment and water column environments—Isotope evidence

Microbial anaerobic oxidation of methane in sediments is a kinetic process associated with a carbon isotope effect which enriches the remaining methane in 13C. Three, models: % residual methane, higher hydrocarbon enrichment, and CO 2-CH 4 coexisting pairs are used to independently calculate fractionation factors (αc) in the range of 1.002–1.014, which overlap the range determined by culture studies, αc is smaller than that associated with methanogenesis by CO 2 reduction or by acetate-type fermentation, and comparison of the coexisting CO 2-CH 4 pairs can distinguish between the formation and consumption processes. Methane oxidation in sediments continues to a threshold concentration of ca. 0.2 mM; the residual methane is either unavailable or unattractive to consumption. Minor amounts of methane may also be produced simultaneously in the methane consumption zone, influencing the apparent fractionation factor in this zone.