Oxygen has three stable isotopes, 16O,
hydrogen has two stable isotopes, 1H
(deuterium), and one radioactive isotope, 3H
(tritium), which is
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
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.
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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:
Isotope Laboratory at University of Colorado
of Isotope Geochemistry at the University
of Arizona, Dept. of Geosciences
Isotope Laboratory at the University of
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Oxygen and hydrogen isotopic ratios are measured
using gas source isotope ratio mass spectrometry
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)
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
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
Although the rate remains constant, it varies
for different isotopic compositions. Thus, with
and again k1
Similarly, with 16O:
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.
The fundamental control on the isotopic composition
of precipitation is temperature. With increasing
temperature, precipitation becomes enriched in
the heavier isotopes,18O
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
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
in meteoric waters as follows:
= 8 d18O
This relationship for 18O
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)
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).
In precipitation, the initial liquid phase of
rain is enriched in 18O
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
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
values (based on Hoefs 1997 and Coplen et al.
This progressive depletion of 18O
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:
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 d18Ovapor
= -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
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:
- get lighter with increasing latitude;
- get lighter toward the continental interior;
- 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
because of the rainout effect.
A map of monthly
values can be accessed on the IAEA site.
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
preferentially concentrate in the liquid phase.
Consequently, in evaporation, water vapor is enriched
whereas the remaining liquid water is enriched
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:
= 14.2 (1-h)
= 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).
can be used to trace the hydrological cycle from
evaporation in the oceans to local precipitation
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
SAHRA's project on the Solute
Balance of the Rio Grande for more information)
Variations in the d18O
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
1) the isotopic component of the event is significantly
different from the pre-event;
2) the event component maintains a constant isotopic
3) vadose water contributions to runoff are negligible,
or vadose water and groundwater are isotopically
4) surface storage contributes only minimally
to the runoff event. (Sklash and Farvolden 1979;
Genereux and Hooper 1998).
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
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
- 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,
- 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,
- 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,
- 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,
- 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.
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John, personal web page at University of
Atomic Energy Agency, Isotope
Hydrology Information System (ISOHIS)
Atomic Energy Agency, The
Development of GNIP (Global Network of Isotopes
Atomic Energy Agency/University of Waterloo,
Maps and Animations
Periodic Table - Hydrogen
Periodic Table - Oxygen
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