Geoscience Reference
In-Depth Information
7.9 Conclusion
Painstaking analysis of the fluctuations in the stable oxygen isotopic composition in
many hundreds of marine sediment cores has revolutionised our understanding of past
climatic fluctuations, notably in global ice volume, ocean temperature and sea surface
salinity. On land, the greatest breakthroughs have come from analyses of
18
O/
16
O
and
13
C/
12
C ratios in speleothems from caves in Eurasia, Africa and the Americas, as
well as from desert lakes across the world. The stable carbon isotopic composition
of calcium carbonate concretions in paleosols provides a record of past changes in
vegetation, notably the proportions of plants following C
3
and C
4
photosynthetic
pathways. Analysis of the C/N ratios in tooth enamel can be used to determine
prehistoric human diets, and a combination of
13
C/
12
C,
15
N/
14
Nand
87
Sr/
86
Sr analyses
can be used to determine the provenance of, for example, elephant ivory used in the
illegal international ivory trade.
87
Sr/
86
Sr analysis of freshwater gastropod shells and
of fluvial and lacustrine clays is a powerful means of reconstructing past hydrologic
and depositional fluctuations and extreme paleoclimatic events, such as the extreme
4.2 ka drought in the north-east quadrant of Africa and beyond. There have been
some useful pioneering attempts to use the Sr/Ca and Mg/Ca ratios preserved in
fossil ostracod shells as a measure of former lake water salinity and temperature,
respectively, although recent studies have stressed the need for greater caution in
using these ratios. These studies point out that there are many factors operating within
lake waters that influence how organisms respond to possible changes in salinity and
temperature.
