Isotope Fractionation

Because isotope fractionation (Section 13.2.2.1) is very small, except for XH and 2H, any effect on the normal chemical behavior of an element is usually ignored. However, when a change in the ratio of two isotopes occurs in some process in the environment, the change can be used to obtain very valuable information about the environment, especially the paleoenvironment. Extremely small changes in the abundance ratio of two isotopes of an element can be measured very accurately by mass spectrometric methods using a variety of ultrasensitive mass spectrometers.

The following are examples of such processes: photosynthesis of complex organic compounds, biological processes, and physical processes such as vaporization of a liquid, diffusion of a gas, and transport across the boundary of two phases. Isotope fractionation may have a temperature coefficient large enough to permit use of the coefficient as a paleothermometer to measure the temperature and temperature changes on the earth's surface (seawater and ice) over a very long period of time or at the time of a major environmental event in the past.

An emerging method for studying the sources and fate of organic pollutants in surface and groundwater systems utilizes compound-specific stable isotope measurements of isotope fractionation of H, C, N, and Cl in compounds accompanying transport, metabolism, and biodegradation. The method uses gas and liquid chromatography and high-precision stable isotope mass spectrometry.

The difference between the isotopic composition of an element in two samples of a material is commonly expressed in terms of the change in the ratio of numbers of atoms of two isotopes (e.g., 18O/16O). When the isotope ratio for an element in a sample of interest is compared with that in a standard sample, the difference between the ratios is usually expressed in terms of delta, 8 (values expressed in "per mil''). For example, 818O is defined by the equation

The following are a few examples of isotope fractionation:

1. Water in leaves of plants is enriched in 18O relative to water in the soil where the plants are growing.

Standard mean ocean water (SMOW) is an example of Rstd for 818O.

Thousands of years before present

Thousands of years before present

FIGURE 14-8 Variation of lsO concentration in ice core samples from the Greenland Ice Sheet for the past 40,000 years. The abrupt change from a cold period to a warm period began about 11,650 years ago. The coldest periods in the North Atlantic are indicated by dots. From K. Taylor, Am. Sci. 87, 320 (1999). Used by permission of Edward Taylor/American Scientist.

FIGURE 14-8 Variation of lsO concentration in ice core samples from the Greenland Ice Sheet for the past 40,000 years. The abrupt change from a cold period to a warm period began about 11,650 years ago. The coldest periods in the North Atlantic are indicated by dots. From K. Taylor, Am. Sci. 87, 320 (1999). Used by permission of Edward Taylor/American Scientist.

2. Photosynthesis in plants results in preferential uptake of 12C so that biomass is depleted in 13C relative to the atmospheric CO2. In limestone,ther-efore, carbon-in-carbon inclusions formed by living organisms differ in their S13C from the inorganic carbon. The value of S13C varies with temperature. As little as 20 pg of carbon is required for mass spectrometric analysis.

3. Nitrous oxide emitted from soil is depleted in 15N and 18O relative to the values for atmospheric N2O.

4. Stratospheric CO2, O3, and O2 are enriched with respect to 18O relative to their isotopic composition in the atmosphere.

5. The isotopic composition of phosphate in vertebrate bone depends on the temperature when the bone was formed. The 16O content is higher at higher temperature.

6. When ice forms in the ocean, it is enriched in 16O relative to water.

7. Both snow and rain are enriched in 18O. The enrichment increases with increasing temperature at the time of precipitation.

8. Atmospheric CO2 is enriched in 14C relative to deep-water CO2.

9. An example of the use of the temperature dependence of S18O to study the temperature profile of ice cores from the Greenland Ice Sheet over the past 40,000 years is shown in Figure 14-8. An abrupt change in climatic conditions occurred about 11,650 years ago. The time when this change began, which has been associated with a change in the behavior of the Gulf stream, can be pinpointed within about 20 years. It is not known whether anthropogenic activities can trigger a change in the way the Gulf stream transports warm water.

10. When atmospheric ozone is formed, the isotopic composition (18O/17O) is not that expected from the mass effect and is referred to as a "mass-independent" effect.14

14Y. Q. Gao and R. A. Marcus, Science, 293, 259 (2001).

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