Geoscience Reference
In-Depth Information
failed to reach the north-west of the continent during this time. Wetter conditions
resumed at 14.2 ka both here and elsewhere in the seasonally wet tropics of northern
Australia.
16.6.2 Diatoms
Diatoms are tiny, single-celled phytoplankton, or, more technically, eukaryotic
microalgae with a siliceous skeleton. They live in lakes, marshes, rivers and the
sea. They are very good indicators of water chemistry, depth and temperature because
many individual species are sensitive to even slight changes in these factors. The
uptake of dissolved silica by diatoms ensures that the siliceous diatom cells, or frust-
ules, accumulate on the lake floor as resistant, usually well-preserved fossils. The
frustules of each diatom species vary in shape, size and ornamentation, very much
like pollen grains. Diatom species are grouped into assemblages and zones, and they
can be used to interpret past environmental changes on the basis of known species tol-
erances towater salinity, temperature and depth (Gasse,
1975
; Gasse,
1980
;Smoletal.,
1986
; Gell,
1997
;Chalie and Gasse,
2002
). Another approach still being developed
is the analysis of the stable oxygen isotopes contained within the silica (SiO
2
)of
the frustules (Leng and Barker,
2006
; Leng and Barker,
2007
; Leng and Sloane,
2008
).
Gasse (
1975
) carried out a pioneering study of the diatom assemblages of Pleis-
tocene and Holocene lakes in the hyper-arid Afar Desert of Ethiopia and was able
to distinguish those lakes fed mainly by surface run-off from those fed only from
groundwater inflow. She found that fluctuations in the first type of lake were a result
of fluctuations in rainfall and evaporation in the upland headwaters, whereas the lakes
fed fromgroundwater were comparatively insensitive to regional climatic fluctuations.
In the Chad Basin, the onset of late Pliocene aridity (Servant,
1973
; Sepulchre
et al.,
2006
) was associated with a diatom flora in the lake deposits that was indicative
of cooler-than-present temperatures (Servant and Servant-Vildary,
1980
). The diatom
and pollen content of Pliocene Lake Gadeb in the south-east uplands of Ethiopia
(see
Chapter 11
) also indicates cooling and progressively drier conditions around
2.5 Ma ago (Williams et al.,
1979
; Gasse,
1980
; Bonnefille,
1983
), when the Northern
Hemisphere ice caps began to expand. This was a time of widespread intertropical
cooling and desiccation that was most likely linked to the expansion of the Northern
Hemisphere ice caps.
Diatoms can often reveal a remarkably detailed picture of past climatic fluctuations
in deserts. The diatom assemblages and stable isotopic composition of Holocene lake
sediments in the northern Sahara show very rapid changes in water chemistry from
fresh to highly saline within this time interval (Fontes et al.,
1985
). Analysis of the
pollen, diatoms, ostracods and stable isotopes preserved within the sediments of five
lakes in the arid Qinghai-Tibetan Plateau region and northern Xinjiang in north-west
