Geoscience Reference
In-Depth Information
18
O values were
similar to those of today but were depleted relative to present between 7.4 ka and
6.9 ka, suggesting a change in atmospheric circulation at that time. He attributed this
change to an increase in the frequency of storms reaching the Negev from north-east
Africa. Modern conditions had resumed by 3.8 ka. (The quoted ages are calibrated
ages; the actual published ages were expressed as
14
C years BP).
seetzeni
from the northern Negev and found that early Holocene
7.8 Trace element geochemistry of ostracods and aquatic snail shells
Initial efforts to unravel the climatic history of desert lakes and wetlands used a
variety of methods, including lake sediments, invertebrate fossils such as mollusc and
ostracod shells, and any associated pollen grains. Ostracods are very small crustaceans
with bivalve shells made of lowMg-calcite. They shed their shells during growth, and
the shells are readily preserved in lake muds. In the early 1980s, a new approach was
pioneered which involved using the trace element geochemistry as well as the stable
isotope analysis of fossil ostracod shells recovered from lake and river sediments
(Chivas et al.,
1986a
;Chivasetal.,
1986b
;Chivasetal.,
1986c
). In essence, the Sr/Ca
ratio in ostracod shells provides a measure of lake water salinity, and the Mg/Ca
ratio provides a measure of lake water temperature. Care needs to be taken to use
ostracods belonging to a single species, because different species may display different
responses to lake water chemistry (Ito et al.,
2003
). If the Sr/Ca values cluster tightly
within a given sediment core, it shows that salinity did not fluctuate much during
the life of the ostracods. If both the Sr/Ca and Mg/Ca values cluster tightly, then
lake salinity and temperature did not fluctuate, indicating deep-water conditions (Ito
et al.,
2003
). If both the Sr/Ca and Mg/Ca values fluctuate widely, salinity fluctuations
are likely, and the water was either shallow or seasonally fluctuating, as in the last
interglacial lakes in the Western Desert of Egypt associated with Middle Stone Age
sites (De Deckker and Williams,
1993
).
Ito and Forester (
2009
) expressed reservations about using the Sr/Ca and Mg/Ca
ratios in ostracod shells as a measure of water temperature and salinity and suggest
that they should be used instead to indicate when changes occur and not why they
occur. Many other factors besides salinity will affect algae and other organisms, such
as ostracods, that live in lakes, so salinity values inferred from Mg/Ca ratios are
best considered as very general estimates to be used in conjunction with estimates
derived from species assemblages of diatoms, ostracods and gastropods, as reviewed
in
Chapter 16
.
Salinity may have a negative influence on the size of aquatic organisms.
Melanoides
tuberculata
shells observed by the author living today in very shallow saline lakes in
the Afar, such as Lake Abhe, were all very small compared to their early Holocene
and Pleistocene counterparts, which flourished when this lake was tens of metres deep
and fresh.
