Geoscience Reference
In-Depth Information
Pole
18
O
snow
=
-30
δ
18
O
rain
=
-20
δ
δ
18
O
rain
=
15
18
O
rain
=
0
δ
δ
18
O
vapor
=
−
9
δ
18
O
seawater
=
0
Equator
Figure 7.9
18
O(in
The atmospheric water cycle and the
) of various types of precipitation. Vapor formed
by evaporation of the warm surface ocean between the tropics migrates with the atmospheric
circulation towards the poles. Incremental precipitation in rain at low latitude and in snow at high
latitude progressively depletes atmospheric water vapor, rain, and snow in
18
O. A similar
poleward depletion of the atmosphere takes place for deuterium with respect to hydrogen,
thereby producing the relationship of
Fig. 7.10
.
δ
kinetic effects during phase changes. This expression yields the equation of the Global
Meteoric Water Line as
18
O
10 (
Fig. 7.10
). Rain waters from temperate regions
and polar ice are depleted with respect to seawater in the light isotope by up to several
percent for oxygen and up to tens of percent for hydrogen. We can infer from the Rayleigh
distillation equation that the amount of precipitation decreases exponentially down the
GMWL: the amount of water reaching the poles is a tiny fraction of the water lost in tran-
sit. This relationship does not pass through the composition of seawater because of kinetic
fractionation during evaporation of warm surface water, while condensation in clouds takes
place at much lower temperatures under near-equilibrium conditions. A somewhat simi-
lar distribution is found with altitude: as air rises in mountain areas, the various types of
precipitation become depleted in D and
18
O.
This relationship implies that almost all underground water is recycled rain water. Geo-
thermal springs lie on a horizontal line through the isotopic composition of the rainfall
of the location. This indicates that the groundwater is rain water in which oxygen has
exchanged its isotopes with the surrounding rock; as the rock is virtually devoid of hydro-
gen,
δ
D
=
8
δ
+
D is left unchanged. Therefore, there is no such a thing as “juvenile” spring water,
i.e. water that comes from the depths of the Earth without ever having seen the surface.
The same is true of hydrothermal springs.
δ
