Geoscience Reference
In-Depth Information
in the last 65 million years, many of which were reconstructed from changes in
marine planktonic assemblages, especially the foraminifera. Another great advantage
of using foraminifera is that they lend themselves to detailed analysis of changes
in the isotopic composition of their calcareous shells. This has allowed subdivision
of the Quaternary Period, identification of individual glacial-interglacial cycles and
the erection of a well-dated marine isotope stratigraphy (Shackleton,
1967
; Shack-
leton,
1977
; Shackleton,
1987
; Lisiecki and Raymo,
2005
; Lisiecki and Raymo,
2007
;
Raymo and Huybers,
2008
), which was discussed in
Chapter 6
.
As with vertebrate fossils, the first step is to establish very precisely the type
of environment in which the present-day foraminifera species of interest are living,
including the optimum habitats as well as the limits. For this, a systematic sampling
programme of ocean water temperature, depth, chemistry and nutrient levels is a
prerequisite for allowing the establishment of 'transfer functions' that correlate species
with particular attributes of their habitats. Such transfer functions may then be applied
to fossil assemblages in order to reconstruct past changes in, for example, sea surface
temperature at particular times in the past. Two problems often arise when using
this approach. One is that the fossil assemblage may have undergone the partial
dissolution of certain species and so will not be fully representative of complete
modern assemblages. The solution is to concentrate on the more robust species least
prone to differential losses from solution. A second and more difficult problem occurs
when the fossil assemblage has no modern counterpart. Here again, a partial solution
is to focus on the particular species which do have modern equivalents.
Only a very limited part of the work on marine microfossils has been carried
out in the immediate vicinity of deserts (Sarnthein et al.,
1982
; Leroy and Dupont,
1994
; Leroy and Dupont,
1997
; van der Kaars and De Deckker,
2002
; van der Kaars
et al.,
2006
). However, perhaps one of the most useful aspects of marine microfossil
studies is that they provide a global context within which to place more local studies.
One important aspect of this idea concerns attempts to reconstruct global patterns
of sea surface temperature during the Last Glacial Maximum (Barrows and Juggins,
2005
), of which the Climate Long-Range Investigation, Mapping and Prediction
(CLIMAP) project is an illustrious early example (McIntyre et al.,
1976
; McIntyre,
1981
). However, probably the greatest contribution of marine micropaleontology to
studies of climatic change in deserts is in its providing of a precise time scale of global
temperature and ice volume changes spanning the entire Cenozoic era (Zachos et al.,
2001
;McGowran,
2005
).
16.4.2 Ostracods, cladocera and chironomids
Ostracods
are tiny crustaceans up to about 1 mm in size with hinged or bivalve shells
made of lowMg-calcite. They shed their shells as they grow, and the shells accumulate
on the lake or swamp floor and become buried in lake or swamp sediments. Some
species of ostracod are tolerant of moderately high levels of water salinity, while others
