Geoscience Reference
In-Depth Information
differences in carbon isotope discrimination in the leaves of twenty-two species of
Nothofagus trees fromacross the SouthernHemisphere. The found that such long-term
and genetically controlled discrimination provided a measure of water use efficiency
by the leaves, and therefore of their comparative ability to adapt to water stress.
One complication can arise as a result of biogenic recycling of atmospheric CO
2
in
certain environments such as flood-plains, leading to carbon isotope gradients varying
across the same ecosystem, both between different species and within the same species
(Martinelli et al.,
1991
).
Further fractionation occurs when animals eat the plants. Bone collagen becomes
enriched by about 5
13
C value of the plants ingested. This
can then indicate what type of plants the animals were eating. In South Africa, where
such studies were pioneered in the late 1970s, browsing animals such as kudu only
feed on leaves of trees and shrubs, which are C
3
plants. The average
‰
relative to the average
13
C value of the
bone collagen of such animals is around
−
21.5
‰
(van der Merwe,
1982
). Grazing
13
C values between
animals, which eat only C
4
grasses, have
−
8
‰
and
−
10
‰
,and
mixed feeders, such as sable antelopes, have values of
.
Using these empirically determined quantitative data, it is possible to use isotopic
measurements on bones and teeth to assess the relative amounts of C
3
and C
4
plants
eaten by prehistoric animals and people (Sealy,
1986
). A refinement of this approach
is to use both carbon and nitrogen bone collagen ratios to determine prehistoric human
diets, a method that can discriminate between cereals, fish and even between camel
herders and goat, sheep and cattle herders (Ambrose and DeNiro,
1986
), although
climate also has an influence on the isotopic composition of bone nitrogen and must
therefore be taken into account (Heaton et al.,
1986
). Using carbon isotopes preserved
in the Miocene/Pliocene fossil fauna of the Potwar Plateau of Pakistan and the Tugen
Hills in northern Kenya, Morgan et al. (
1994
) were able to show that C
4
grasses had
first appeared in the animal diet by 9.4 Ma and were present in Kenya by 15.3 Ma,
although not as a major component of herbivore diet until 7 Ma.
One practical by-product of this work has been the ability to use a combination
of isotope ratios (
13
C/
12
C,
15
N/
14
Nand
87
Sr/
86
Sr) to determine the source areas of
elephant ivory, thereby providing a powerful means to help control the illegal trading
in ivory (van der Merwe et al.,
1990
;Vogeletal.,
1990
).
Ayliffe and Chivas (
1990
) investigated the oxygen isotopic composition (
−
13
‰
to
−
15
‰
18
O)
18
O
p
) in modern kangaroos and wallabies (Macropods) from a
variety of climatic zones across Australia. They found a strong correlation between
of bone phosphate (
18
O
p
and mean annual environmental relative humidity. Since Macropods consume
substantial amounts of plant leaf water, the
18
O
p
probably reflects leaf water frac-
tionation processes, which are in turn controlled by relative humidity. They concluded
that
18
O
p
from fossil Macropod bones could be used as a measure of past changes
in humidity. On the face of it, this seems to be a reasonable conclusion. However,
the stability of apatite phosphate in fossil bones and teeth, long considered resistant
