Geoscience Reference
In-Depth Information
the Old Kingdom in Egypt brought about by the catastrophic drought centred around
4.2 ka and evident as far away as Mesopotamia and the Indus Valley. In strong contrast
to the Egyptian Old Kingdom, the Kerma civilization in the Nile Valley of northern
Sudan survived this event (evident in the strontium ratios in local Nile alluvium) and
persisted for a further thousand years before succumbing to invasion from Egypt and
a further decline in Nile flow (Macklin et al.,
2013
).
Other uses of strontium isotopes include determining the sources of alluvial clays,
calcareous dust (Dart et al.,
2004
;Dartetal.,
2007
), volcanic ash, wood, shells
and bones, including fish vertebrae. In the case of wind-blown dust, strontium isotope
analysis is often combined with analysis of the neodymium isotope ratios
143
Nd/
144
Nd
to pinpoint dust provenance more exactly (Dart et al.,
2007
). Chen et al. (
1999
)
compared the Rb/Sr ratios measured in two loess profiles in central China covering
the last 130 ka with the marine SPECMAP
18
O curve of Imbrie et al. (
1984
), which
is a measure of changes in global ice volume, as explained in
Section 7.3
. They found
a close correlation between the two and concluded that the Rb/Sr ratio is a sensitive
indicator of changes in the East Asian monsoon associated with changes in global ice
volume.
Initial determination that the late Quaternary volcanic ash found in the Son Valley
of north-central India by Williams and Royce (
1982
) had come from the most recent
(74 ka) eruption of Toba volcano in Sumatra arose from a comparison of the
87
Sr/
86
Sr
values preserved within the ash (Williams and Clarke,
1995
) with those obtained by
Whitford (
1975
) from welded tuffs around the parent caldera.
7.7 The carbon isotope record in fossil plants, soils, bones and teeth
During photosynthesis, plants absorb carbon dioxide from the atmosphere and, under
the influence of sunlight and the plant enzyme chlorophyll, convert the CO
2
to starch,
which is then used for plant growth. Plants fix atmospheric carbon dioxide in one
of three different ways. All trees, most shrubs and those grasses that grow in shaded
forests or in temperate regions follow the Calvin, or C
3
, pathway of photosynthesis
(van der Merwe,
1982
; Cerling et al.,
1991
). Grasses adapted to growing in strong
sunlight, such as most tropical grasses, follow the Hatch-Slack, or C
4
. photosynthetic
pathway. Most succulent plants follow the third, or CAM, pathway, so named because
it involves crassulacean acid metabolism, abbreviated to CAM. All three systems of
photosynthesis fractionate the carbon isotope ratio of atmospheric carbon dioxide in
quite different ways (Vogel,
1978
;Vogeletal.,
1978
; van der Merwe,
1982
).
As a result of this differential carbon isotopic fractionation during photosynthesis,
C
4
plants tend to have
13
C values between
−
9
‰
and
−
16
‰
(mean:
−
12.5
‰
), C
3
13
C values between
plants have
−
20
‰
and
−
35 per mil
‰
(mean:
−
16.5
‰
)and
13
C values averaging about
CAM plants have
(van der Merwe,
1982
).
In controlled environment growth cabinets, Read and Farquhar (
1991
) examined the
−
16.5
‰
