Oxygen has three stable isotopes. Natural variation of the oxygen isotopic composition of water, when combined with hydrogen isotopes, can be used for determining precipitation sources as well as evaporation effects. In addition the oxygen isotope ratio of solid phases (e.g. carbonate minerals) can record paleo-climate and paleo-hydrologic information.

• Water Isotopes
• Cost of Analysis
• Measurement Techniques
• Fractionation During Precipitation
• Fractionation During Evaporation
• Hydrological Applications
• References and further reading
• Internet resources

Water Isotopes (return to top)

Oxygen has three stable isotopes, ¹⁶O, ¹⁷O, and ¹⁸O; hydrogen has two stable isotopes, ¹H and ²H (deuterium), and one radioactive isotope, ³H (tritium), which is discussed separately.

Oxygen and hydrogen are found in many forms in the earth's hydrosphere, biosphere, and geosphere. Oxygen is the most abundant element in the earth's crust. Hydrogen also is common in the biosphere and is a constituent of many minerals found in the geosphere. Most importantly, oxygen and hydrogen combine to form water, thus making their isotopic composition a powerful tracer of the hydrosphere.

There are nine isotopic configurations for water, which are distinguished by their mass numbers as well as their characteristics. However, because of the low abundance of the heavier isotopes, almost all water molecules are of three isotopic combinations.

Characteristics of three types of water molecules:

(excerpted from Hoefs 1997, p. 4)

Models of oxygen/hydrogen isotopic fractionation have been developed and refined over the last century. Models of isotopic variability take into account vapor pressure, humidity, temperature, altitude, rainout and moisture content, evaporation and solute concentration, and combinations thereof. Because of their close relationship and the abundance and importance of water on the planet, O and H and their isotopic fractionation characteristics are usually considered together. As a result of fractionation, waters develop unique isotopic compositions that can be indicative of their source or the processes that formed them. Isotopic composition differs for sea water, polar ice, atmospheric water vapor, and meteoric water.

Cost of Analysis (return to top)

Approximately $20 to $35 per water sample.

Laboratory analysis of oxygen isotopic composition of solids such as OH^- minerals, organic solids, silicate oxides, and carbonates range from $25 to $160 per element.

See the following for more information:

Geochron Labs

Zymax Isotope Laboratory

Stable Isotope Laboratory at University of Colorado

Laboratory of Isotope Geochemistry at the University of Arizona, Dept. of Geosciences

Environmental Isotope Laboratory at the University of Waterloo

Measurement Techniques (return to top)

Oxygen and hydrogen isotopic ratios are measured using gas source isotope ratio mass spectrometry (IRMS).

Oxygen and hydrogen compositions are reported as delta values. Delta values (d) are commonly used in light stable isotope geochemistry to express isotopic composition in terms of per mil (‰) deviation from a standard. Stable isotope ratios of deuterium/hydrogen (²H/¹H) and ¹⁸O/¹⁶O of water are conventionally expressed as a per mil (‰) deviation from SMOW (Standard Mean Ocean Water) or VSMOW (Vienna SMOW). In carbonates, the isotopic composition of oxygen is sometimes compared instead to PDB, a standard based on the Peedee Formation, a carbonate rock found in South Carolina.

Fractionation During Precipitation (return to top)

Equilibrium fractionation describes isotopic exchange reactions that occur between two different phases of a compound at a rate that maintains equilibrium, as with the transformation of water vapor to liquid precipitation.

For water in a closed system, equilibrium fractionation can be expressed as:

where v is the water vapor phase, l is the liquid phase, and the rate of exchange is constant (k₁ = k₂). Although the rate remains constant, it varies for different isotopic compositions. Thus, with ¹⁸O:

and again k₁ = k₂.

Similarly, with ¹⁶O:

and .

However, ; rather, such that ¹⁸O/¹⁶O_(v)<¹⁸O/¹⁶O_(l); thus, ¹⁸O becomes enriched in the liquid and ¹⁶O becomes enriched in water vapor. The process is in equilibrium in both instances, but the rate of these exchanges is different, so that enrichment of one of the isotopes results.

Temperature effect

The fundamental control on the isotopic composition of precipitation is temperature. With increasing temperature, precipitation becomes enriched in the heavier isotopes,¹⁸O and ²H, in a linear relationship. Temperature affects fractionation at a rate of approximately 0.5‰ for every C° for oxygen. Similar effects are shown with increasing elevation and increased distance from the equator (both of which correspond to lower temperature). Because precipitation becomes progressively enriched in light oxygen as it moves toward the cold polar regions, polar ice constitutes a reservoir of ¹⁶O enriched water as compared to sea water.

Precipitation and equilibrium fractionation

The dD and d¹⁸O values for precipitation worldwide behave predictably, falling along the global meteoric water line (GMWL) as defined by Craig (1961b), who expresses the relationship between ¹⁸O and ²H in meteoric waters as follows:

d²H = 8 d¹⁸O +10‰

This relationship for ¹⁸O and ²H isotopes is primarily a reflection of differences in their equilibrium fractionation factors. The slope of the GMWL expresses this ratio, which is eight times greater for oxygen than hydrogen.

The validity of the model is shown in Rozanski's (1993) compilation of average annual values for these isotopes (as plotted against the GMWL) for precipitation monitored at stations throughout the IAEA (International Atomic Energy Agency) global network:

Figure 1. (Clark and Fritz 1997, p. 37, as compiled in Rozanski et al. 1993, modified by permission of American Geophysical Union).

Local meteoric water lines in arid environments will exhibit the same slope, but plot higher in relation to d²H because of increased evaporation. Likewise, LMWLs of humid environments maintain the slope of 8, but the line shifts toward increased d¹⁸O because the phase change tends toward liquid precipitation. (See Figure 6 below).

Rainout effect

In precipitation, the initial liquid phase of rain is enriched in ¹⁸O and ²H as compared to the later precipitation. Consequently, in rain events, the precipitation gets lighter as the rain continues, a phenomenon known as "rainout effect" or "amount effect." Similarly, the center of a large land mass or continent has precipitation that is depleted in ¹⁸O and ²H, a phenomenon known as the "continental effect." Isotopically enriched rain forms and falls from a diminishing vapor mass, and the residual vapor becomes isotopically depleted with respect to earlier rains from the same cloud. Rainout consequently evolves to colder, isotopically-depleted precipitation: cold climates plot at the depleted end of the GMWL and warm environments plot at the upper end.

Figure 2. Rainout effect on d²H and d¹⁸O values (based on Hoefs 1997 and Coplen et al. 2000).

This progressive depletion of ¹⁸O and ²H is typically described using a Rayleigh distillation. The Rayleigh equation applies not only to the temperature-isotope evolution during rainout, but the progressive partitioning of heavy isotopes into a water reservoir as it diminishes in size:

where R_o is the initial isotope ratio, R is the ratio after the process occurs, f is the residual component, and a is the equilibrium fractionation factor.

Figure 3. Change in the ¹⁸O content of rainfall according to a Rayleigh distillation, starting with d¹⁸O_vapor = -11‰, temp. = 25°C, and final temp. of -30°C. Note that at 0°C, fractionation between snow and water vapor replaces rain-vapor fractionation. The fraction remaining has been calculated from the decresase in moisture carrying capacity of air at lower temperatures, starting at 25°C. Dashed lines link d¹⁸O of precipitation with temperature of condensation. (Reproduced with permission from Clark and Fritz 1997, p.48)

Patterns of Isotopic Composition

Combined effects of temperature and rainout on a continental scale are evident in the following illustration (modified from Kharaka and Thordsen 1992, and Taylor and Margaritz 1978), which shows average dD of meteoric water in North America.

Figure 4. Average dD of precipitation in North America.

Note that the values:

1. get lighter with increasing latitude;
2. get lighter toward the continental interior;
3. demonstrate sharp changes in the mountain areas, notably the Sierra Nevada of California. Apart from temperature/elevation effects, the windward side of a range receives precipitation enriched in ¹⁸O and ²H because of the rainout effect.

A map of monthly global d¹⁸O values can be accessed on the IAEA site.

Fractionation During Evaporation (return to top)

Kinetic fractionation is similarly related to mass differences between the nuclei of isotopes, but is associated with incomplete and unidirectional processes such as evaporation and diffusion. In general, the lighter isotope will react faster and will become concentrated in products.

For water, the higher the mass number, the lower the vapor pressure. Thus, ¹⁶O and ¹H preferentially enter the vapor phase, whereas ¹⁸O and ²H preferentially concentrate in the liquid phase. Consequently, in evaporation, water vapor is enriched in ¹⁶O and ¹H, whereas the remaining liquid water is enriched in ¹⁸O and ²H. More specifically, H₂¹⁸O is enriched in liquid water by 1% relative to its concentration in water vapor at the same temperature.

Factors such as humidity, salinity, and temperature affect kinetic fractionation of water during evaporation. The effect of humidity on isotopic enrichment in evaporating water can be expressed as follows:

10³lna¹⁸O_l-v = 14.2 (1-h)‰

10³lna²H_l-v = 12.5 (1-h)‰

where h is relative humidity. The lower the relative humidity, the faster the evaporation rate and the greater the kinetic fractionation. Humidity affects oxygen and hydrogen differently such that the slope of the evaporation line will vary due to changes in relative humidity. At very low relative humidities (< 25%) the slope of the evaporation line will be close to 4; for moderate relative humidities (25% to 75%) the slope will be between 4 and 5; only for relative humidities above 95% does the slope approach 8, the slope of the meteoric water line (Clark and Fritz 1997). (See Figure 6 below).

For example, evaporation from rivers and reservoirs in arid regions often exhibits deviations from local meteoric water lines, and serves as an example of kinetic fractionation.

Figure 5. Rivers in humid regions (Amazon) show little deviation form the GMWL; those flowing in arid lands (Rio Grande in the southwestern U.S. and the Darling River in Australia) do.

Likewise, relative humidity affects the isotopic composition of the water vapor. This is primarily reflected in the d value, or deuterium excess, of the meteoric water line. If the vapor source region for precipitation is arid (low humidity), d values will be high, upwards of 20. In contrast, d values for humid regions will be low, approaching 0.

Figure 6. Summary diagram of how hydrologic processes affect oxygen and hydrogen isotopic composition of water.

For more information, discussion, and relevant links to publications, see John Gibson's web page (University of Waterloo).

Hydrological Applications (return to top)

d¹⁸O and d²H can be used to trace the hydrological cycle from evaporation in the oceans to local precipitation and groundwater.

Because d¹⁸O and d²H values have a strong positive correlation with temperature, their measurements in ice cores are valuable indicators of climate variability. Values can be used to date snow and determine average snow accumulation rates.

Water isotopes can be used to determine open water evaporation from rivers, reservoirs, and lakes.
(See SAHRA's project on the Solute Balance of the Rio Grande for more information)

Variations in the d¹⁸O and d²H values of precipitation can be used to differentiate relatively "old" (uniform) water and the more variable "new" water to determine their respective contributions to a stream during periods of storm runoff. Two-component hydrographs are used in this analysis to distinguish "event" and "pre-event" isotopic composition of streamflow. Successful hydrograph separation requires that:

1) the isotopic component of the event is significantly different from the pre-event;
2) the event component maintains a constant isotopic content;
3) vadose water contributions to runoff are negligible, or vadose water and groundwater are isotopically equivalent;
4) surface storage contributes only minimally to the runoff event. (Sklash and Farvolden 1979; Genereux and Hooper 1998).

With SiO₂, d¹⁸O has been used to show multiple contributions to a streamflow: runoff, soil water, and groundwater (Hinton et al 1994). Similarly, DeWalle et al. (1988) have used a three-component model to separate storm streamflow into groundwater, soil water and channel precipitation, using a calculation of the rate of channel precipitation as the product of the throughfall rate and the surface area of the stream.

Isotopic composition studies can be used to trace water use, uptake, and transport by plants. Evaporative processes cause significant isotopic enrichment in a plant's xylem-sap and leaves at a magnitude dependent on leaf transpiration rate, humidity gradient, and the isotopic composition of atmospheric water.

References and further reading (return to top)

• Coplen, T.B., A.L. Herczeg, and C. Barnes, Isotope engineering: using stable isotopes of the water molecule to solve practical problems, in Environmental Tracers in Subsurface Hydrology, ed. by P.G. Cook and A.L. Herczeg, Kluwer Academic Publishers, Boston, 2000.



• Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis Publishers, Boca Raton, 1997.



• Craig, H., Standard for reporting concentrations of deuterium and oxygen-18 in natural waters, Science, 133, 1833-1934, 1961a.



• Craig, H. Isotopic variations in meteoric waters, Science, 133, 1702-1703, 1961b.



• Craig, H., and L.I. Gordon, Deuterium and oxygen-18 variations in the ocean and the marine atmosphere, in Stable Isotopes in Oceanographic Studies and Paleotemperatures, Soleto, July 26-27, 1965, Consiglio Nazionale delle Richerche, Laboratorio di Geologia Nucleare, Pisa, 1-22, 1965.



• DeWalle, D.R., B.R. Swistock, and W.E. Sharpe, Three component tracer model for stormflow on a small Appalachian forested catchment, J. of Hydrol, 104: 301-310, 1988.



• Faure, G., Principles of Isotope Geology, 2nd ed., John Wiley and Sons, New York, 1986.



• Gat, J.R., The isotopes of hydrogen and oxygen in precipitation, in Handbook of Environmental Isotope Geochemistry, vol. 1A, edited by P. Fritz and J.C. Fontes, pp. 21-47, Elsevier, Amsterdam, 1980.



• Genereux, D.P. and R.P. Hooper, Oxygen and hydrogen isotopes in rainfall-runoff studies, in Isotope Tracers in Catchment Hydrology, ed. by C. Kendall and J.J. McDonnell, pp. 319-346, Elsevier, Amsterdam, 1998.



• Gonfiantini, R, Environmental isotopes in lake studies, in Handbook of Environmental Isotope Geochemistry, vol. 2, edited by P. Fritz and J.-Ch. Fontes, pp. 113-168, Elsevier, Amsterdam, 1986.



• Hinton, M.J., S.O. Schiff, and M.C. English, Examining the contribution of glacial till water to storm runoff using two and three-component hydrograph separations, Water Resour. Res., 30, 983-993, 1994.



• Hoefs, J., Stable Isotope Geochemistry, 4th ed., Springer-Verlag, Berlin, 1997.



• Rozanski, K., L. Araguás-Araguás, and R. Gonfiantini, Isotopic patters in modern global precipitation, in Climate Change in Continental Isotopic Records, Geophys. Monogr. Ser., 78, ed. by P.K. Swart, et al, pp. 1-36, AGU, Washington, DC, 1993.



• Sklash, M.G., and R.N. Farvolden, The role of groundwater in storm runoff, J. of Hydrol. 43: 45-65, 1979.



• Sklash, M.G., R.N. Farvolden, and P. Fritz, A conceptual model of watershed response to rainfall, developed through the use of oxygen-18 as a natural tracer, Can. J. Earth Sci., 13, 271-283, 1976.

Internet resources (return to top)

Gibson, John, personal web page at University of Waterloo

International Atomic Energy Agency, Isotope Hydrology Information System (ISOHIS)

International Atomic Energy Agency, The Development of GNIP (Global Network of Isotopes in Precipitation)

International Atomic Energy Agency/University of Waterloo, GNIP Maps and Animations

USGS Periodic Table - Hydrogen

USGS Periodic Table - Oxygen

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