WHAT CAN WE LEARN FROM ISOTOPES? - Chemical Fundamentals of Geology and Environmental Geoscience - page 196

Table 10.3 Summary of low- A stable isotope systems used in Earth and environmental science (see Figure 10.9)

Element Isotopes Isotope ratio used Standard used Applications*

Hydrogen

1 H, 2 H (=D) 2 H/ 1 H = D/H

VSMOW § Hydrothermal water-rock interactions,

water provenance (Figs. 10.10a,b),
palaeoclimates (Figs. 10.11, 10.12),
biochemical processes

Carbon

12 C, 13 C 13 C/ 12 C

VPDB ¶ Composition of Earth's early

atmosphere, detection of early life
(Fig. 10.13), mantle heterogeneity
and origins of diamonds

Nitrogen

14 N, 15 N 15 N/ 14 N

Atmospheric N 2 gas Oceanic nitrate utilization, mixing of

fresh and marine waters

Oxygen

16 O, 17 O, 18 O 18 O/ 16 O

VSMOW §
VPDB ¶

Oceanic palaeotemperatures
(Figs. 10.11b, 10.12),
geothermometry, hydrothermal
water-rock interaction, water
provenance (Figure. 10.10)

Sulphur

32 S, 33 S, 34 S, 36 S 34 S/ 32 S

Troilite (FeS) from the Canyon
Diablo iron meteorite

Origins of sulfide ores, Earth
atmosphere evolution (Fig. 10.14)

*After Henderson and Henderson (2009).
§ 'Vienna Standard Mean Ocean Water' - despite its name, a pure water sample having specific D/H and 18 O/ 16 O abundance ratios, adopted
by the International Atomic Energy Agency (IAEA) in Vienna in 1968.
¶ 'Vienna Peedee belemnite' is a similar artificial benchmark for 13 C/ 12 C adopted by the IAEA in 1985, based on belemnite fossil carbonate
from the Peedee Formation in South Carolina.

Hydrogen consists of two stable isotopes, 1 H and 2 H
(Figure 10.9). It is the only chemical element whose iso-
topes are distinguished by separate chemical names.
2 H - whose nucleus comprises one proton and one
neutron - is known as deuterium (from the Greek
deut eros meaning 'second'), and the chemical symbol D
is sometimes used for it in place of 2 H. Hydrogen also
has a third isotope 3 H - known as tritium (1 proton +
2 neutrons) - which is radioactive with a half-life of
12.3 years.
Oxygen consists of three stable isotopes, 16 O, 17 O and
18 O (Figure 10.9).

rain depletes the H 2 O vapour remaining in the atmos-
phere still further (Figure 10.10a).
It follows that moist subtropical air masses, depos-
iting rain as they migrate to higher latitudes and
lower temperatures, experience progressive depl-
etion in HDO and H 2 18 O (Figure 10.10a). Accordingly
the isotopic composition of rain and snow (and fresh
waters derived from them) is found to correlate
strongly with latitude (Figure 10.10b). A similar trend
may be seen with distance from the ocean towards
continental interiors. On the other hand, equatorial
freshwater bodies subject to high evaporation rates
(such as rivers and lakes in East Africa) may exper-
ience enrichment in HDO and H 2 18 O as shown in
Figure 10.10b.
H and O isotope ratios enable us to recognize three
distinct categories of water that can be involved in
geological reactions (Figure 10.10b):

The terrestrial water cycle
The H 2 18 O molecule, being 12% heavier than H 2 16 O, is
slightly more difficult to evaporate: its vapour pres-
sure at 100 °C is 0.5% lower than that of H 2 16 O, causing
its boiling point to be 0.14 °C higher. Vapour in equilib-
rium with water is therefore slightly deficient in both
H 2 18 O and the other 'heavy' water molecule HDO
( 1 H 2 H 16 O) relative to the coexisting liquid phase.
Atmospheric water vapour, being produced by evap-
oration of seawater, is thus measurably depleted in
these heavier molecules. Furthermore, precipitation of

(a) Seawater, with δ D and δ 18 O close to zero.
(b) Rain-derived ( meteoric ) surface- and groundwater,
having variable (but correlated) negative δ D and
δ 18 O values related to latitude of deposition
('GMWL', Figure 10.10b).