The isotopic ratios of plant material integrate both biotic
and abiotic factors that fractionate between different isotopes based
upon their atomic masses. These factors fractionate between different
pools in the hydrologic cycle, which results in each pool having a unique
isotopic signature (Fig. 1). Thus, knowing the isotopic signatures of
different water sources can help plant physiologists determine the origin
from which plant water has been derived.
Fig. 1 Using tritium 3H and d 18O to identify pools
in the hydrologic cycle. Tritium, released into the atmosphere during
the testing of the hydrogen bomb, is expressed in tritium units (TU)
and del 18O is expressed conventionally as per mil (figure obtained
from IAEA).
Tracing the Hydrologic Cycle
The relationship between the d18O and d2H in the world's fresh waters
follows a predictable linear relationship (Fig 2). This predictable
relationship is referred to as the "meteoric water line" (Craig,
1961) and allows for the approximation of d2H based on the measurement
d18O and vice versa. However, deviations from this predictable relationship
can reveal perturbations to the hydrologic cycle. Millenial variations
in d18O contained in the world's oceans and the polar ice caps have
been used to infer changes in past climate (Shakleton
and Opdyke, 1973). Because of their different masses, isotopes have
different kinetics that are exemplified during phase changes, which
result in fractionation.
Fig. 2 The "meteoric water line" as calculated
by Craig
(1961), with warm regions characterized by more enriched (positive)
values of hydrogen and oxygen isotopes and cooler regions characterized
by more depleted (negative) values. Isotope ratios are reported
with respect to Standard Mean Ocean Water (smow)
Evaporation
As heat is applied to liquid water, it changes to a gaseous
state, and this process discriminates between different isotopes. This
is conceptually intuitive, as more energy is required to lift and transport
a heavier atom than a light atom. This is why salinity and d 18O are
positively correlated in the world's oceans- as salinity increases so
does the amount of the heavier isotope (Craig
and Gordon, 1965). However, this relationship is affected by humidity,
whereby water interacts with the vapor phase. If we assume that relative
humidity is zero (h = 0), then the enrichment of both d 18O and d 2H,
follow the exponential Rayleigh distillation function:
R=Ro f ^(a-1)
Where, R represents the isotope ratio as a function of the fraction
remaining (f), a is the equilibrium fractionation constant of evaporation
and Ro represents the initial isotopic ratio. However, as atmospheric
humidity approaches 100%, d 2H will become more depleted than d 18O
during evaporation.
Precipitation
The fractionation imparted to water as it changes from a gaseous state
to a liquid state is very similar to that of evaporation except the
reverse. In order for water to condense and precipitate from an air
mass, the temperature must drop. As temperature drops, the heavier isotopes
are selectively precipitated from the air mass through distillation
(Eq 1). This phenomenon is often referred to as the "rainout"
effect (Dansgard,
1964), which describes the successive depletion of d 18O and d 2H
as an air mass travels from its source and loses heavier isotopes through
precipitation (Fig. 3). This effect can be enhanced by orographic effects
that may cause an abrupt cooling of an air mass as it rises and thus
a strong gradient in isotopic ratios.
Fig. 3 Graphical depiction of Rayleigh distillation,
whereby heavier isotopes of oxygen are selectively precipitated from
an air mass as temperature decreases.
Groundwater
Most plants obtain their water through their roots from the groundwater.
Thus knowing the isotopic signature of groundwaters at different depths
can be very informative for plant physiologists. Water found deeper
in the Earth's crust tends to be older and have a more attenuated isotopic
signal. This means that deep groundwater exhibits much less seasonal
variability in its constituent isotopes than water at the surface. This
is intuitive, as waters at depth are not subject to the same seasonal
temperature variability, which induces precipitation and evaporation-
two important fractionating phenomena.
Global Distributions of Isotopes
The physical processes of evaporation and condensation impart isotopic
signatures to water as it circulates through the hydrologic cycle. Understanding
these basic physical principles allows us to make basic predictions
about the isotopic signatures of waters of certain regions of the globe.
For instance, as we increase in altitude and the temperature decreases,
the isotopic signature of source water will become depleted (more negative).
An increase in latitude will also cause a depletion in the isotopic
ratio of source water due to decreased insolation and colder temperature
towards the poles. These processes in conjunction with localized meteorological
phenomena ultimately determine isotopic signatures of source waters
available to plants (Fig.4).
Fig 4. Distribution of del 18 O in global precipitation
(obtained from IAEA)
