Isotope fractionation

About: Isotope fractionation is a research topic. Over the lifetime, 4138 publications have been published within this topic receiving 209991 citations.

Empirical Relationships Between Elevation and the Stable Isotope Composition of Precipitation and Surface Waters: Considerations for Studies of Paleoelevation Change

Abstract: A compilation of 68 studies from throughout many of the world9s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects. The isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the Himalayas and at elevations >5000 m. There are no significant differences in isotopic lapse rates from most regions of the world (∼0.28 permil/100 m) except at the extreme latitudes where isotopic lapse rates are higher. Given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals, this global isotopic lapse rate can be used to place numerical constraints on the topographic development of some ancient mountain belts or plateaus. There are many complicating factors that can confound interpretation of paleoelevation change based on stable isotopes, and many of these are unique to specific mountain belts or time periods. Relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedogenic or authigenic mineral and the water from which it forms. In cases where isotopic proxy minerals are sampled from localities where temperature will change simultaneously with elevation change, the apparent change in the isotopic composition of precipitation may be dampened by several permil. This suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau.

Generation, Accumulation, and Resource Potential of Biogenic Gas

Abstract: Biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms. More than 20% of the world's discovered gas reserves are of biogenic origin. A higher percentage of gases of predominantly biogenic origin will be discovered in the future. Biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread, large quantities at shallow depths. In rapidly accumulating marine sediments, a succession of microbial ecosystems leads to the generation of biogenic gas. After oxygen is consumed by aerobic respiration, sulfate reduction becomes the dominant form of respiration. Methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted. The most important mechanism of methane generation in marine sediments is the reduction of CO2 by hydrogen (electrons) produced by the anaerobic oxidation of organic matter. CO2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures. The factors that control the level of methane production after sediment burial are anoxic environment, sulfate-deficient environment, low temperatu e, availability of organic matter, and sufficient space. The timing of these factors is such that most biogenic gas is generated prior to burial depths of 1,000 m. In marine sediments, most of the biogenic gas formed can be retained in solution in the interstitial (pore) waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column. Under certain conditions of high pressures and (or) low temperatures, biogenic methane combines with water to form gas hydrates. Biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses. The hydrocarbon fraction of biogenic gas consists predominantly of methane. The presence of as much as 2% of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low-temperature degradation of organic matter. The amounts of hydrocarbon components other than methane generally are proportional to temperature, age, and organic-matter content of the sediments. Biogenic methane is enriched in the light isotope 12C (^dgr13C1 lighter than -55 ppt) owing to kinetic isotope fractionation by methanogens. The variations in isotopic composition of biogenic methane are controlled primarily by ^dgr13C of the original CO2 substrate, which reflects the net isotopic effect of both addition and removal of CO2. The methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas. The possible complicating factors require that geologic, chemical, and isotopic evidence be considered in attempts to interpret the origin of gas accumulations. Accumulations of biogenic gas have been discovered in Canada, Germany, Italy, Japan, Trinidad, the United States, and USSR in Cretaceous and younger rocks, at less than 3,350 m of burial, and in marine and nonmarine rocks. Other gas accumulations of biogenic origin have undoubtedly been discovered; however, data that permit their recognition are not available.

On the enrichment of H2 18-O in the leaves of transpiring plants.

Abstract: The vapor pressure difference between H2 18O and H2 16O is the reason for the accumulation of the heavy molecule in transpiring leaves. Since photosynthesis on land is the main source of atmospheric oxygen, this mechanism is important for the remarkable enrichment of18O in atmospheric O2 (Dole effect). Using a simple box model for transpiring leaves a quantitative understanding of the isotope fractionation is possible which is well confirmed by the results of model experiments as well as by measurements on trees. Maximum enrichment of H2 18O in the water of leaves (relative to soil water) is 25 ‰ (theoretically, for dry air) and was found under natural conditions to be 21 ‰ (for 28 % relative humidity); minimum theoretical enrichment is zero (observed 2.5 ‰).

The oxygen isotope and cation exchange chemistry of feldspars

Abstract: Cation exchange experiments between alkali and alkaline-earth feldspars and corresponding 2-3 molal aqueous chloride solutions were performed at a fluid pressure of one kilobar over the temperature range 350°-800°C. Oxygen isotope analyses of the exchanged feldspar indicate that essentially complete oxygen isotope equilibration between solution and feldspars accompanies the cation exchange. Oxygen isoptope fractionations obtained this way were proved to be equilibrium fractionations by their agreement with those obtained by true isotope exchange reactions between synthetic feldspars and pure water. The oxygen isotope fractionation factor (alpha) between alkali feldspar and water in the temperature range studied is given by the expression 10 3 ln alpha=2.91 (10 6 T -2 )-3.41. No isotope fractionation was discernible between albite and potassium feldspar. However, the alkali feldspars were found to concentrate O 18 relative to the alkaline-earth feldspars, indicating a relationship between the Al/Si ratio in feldspar and tendency to concentrate O 18 . The plagioclase-water fractionation follows the equation: 10 3 ln alpha=(2.91-0. 76beta)(10 6 T -2 )-3.41-0.41beta where beta is the An content of plagioclase. Observations made during the course of this work suggest that the mechanism of oxygen and cation exchange in these experiments involves fine-scale solution and redeposition in a fluid film at the interface between exchanged and unexchanged feldspar. A mechanism involving simple solid-state diffusion cannot explain the observed communication between the oxygen at the interfacial boundary and the solution.

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes

Abstract: The oxygenation of Earth's atmosphere is thought to have occurred in two broad steps, but details of the process remain uncertain. Frei et al. use stable chromium (Cr) isotopes from banded iron formations, which are sedimentary rocks containing large amounts of oxygen as iron oxides, to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation of Earth's atmosphere–hydrosphere system. Their data suggest a transient elevation in atmospheric and surface ocean oxygenation prior to the first great rise of oxygen 2.45 to 2.2 billion years ago (the Great Oxidation Event). Chromium is not fractionated in 1.88-billion-year-old banded iron formations, indicating a decline in atmospheric oxygen. The authors propose that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. It is thought that oxygenation of the Earth's atmosphere occurred in two broad steps, but details of the evolution of atmospheric oxygenation remain uncertain. Chromium (Cr) stable isotopes from banded iron formations are now used to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth's atmosphere–hydrosphere system. Geochemical data1,2,3,4 suggest that oxygenation of the Earth’s atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between ∼2.45 and 2.2 Gyr ago1,5, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era (∼800–542 Myr ago)3,4, ultimately leading to oxygenation of the deep ocean ∼580 Myr ago3, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth’s atmosphere–hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters ∼2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45–2.2 Gyr ago (the Great Oxidation Event)1,5. In ∼1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (δ53Cr up to +4.9‰), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life3,4,6.