Geoscience Reference
In-Depth Information
carbonates themselves, which often only give a very generalised climatic signal (Abell,
1985
; Williams et al.,
1987
; Abell and Williams,
1989
; Eyles and Schwarcz,
1991
;
Abell et al.,
1996
; Ayliffe et al.,
1996
; Abell and Hoelzmann,
2000
; Glasby et al.,
2007
).
As a precursor to analysing the isotopic ratios in freshwater gastropod shells from
the Sahara, Nile Valley and East Africa, Abell (
1985
) compiled the isotopic ratios
in modern gastropod shells from more than eighty localities throughout Africa. The
analysed values showed consistent variations with latitude, elevation and the amount
of rainfall, in good accord with previous compilations of the isotopic composition
of rainfall across the globe (Dansgaard,
1964
; Yurtsever,
1975
). Abell and Williams
(
1989
) examined the oxygen and carbon isotope ratios in gastropod shells recovered
from Holocene lake and spring sediments in the southern Afar and Ethiopian rifts.
They also examined isotopic changes along the growth whorls of
Melanoides tuber-
culata
shells living in Lake Lyadu, a small lake in the southern Afar, which dried
out soon after the shells had been collected. The fluctuations revealed intervals of
little or no growth, probably as a result of a sharp drop in lake level during the recent
droughts in that area. Results from the other Ethiopian samples were consistent with
the known history of lake level fluctuations in that region, and they revealed a clear
discrimination between gastropods that had lived in ponds fed primarily from local
springs and lakes in receipt of far-travelled run-off from the Ethiopian Highlands.
Fontes et al. (
1985
) investigated the diatom assemblages and stable isotopic com-
position of Holocene lake sediments in the northern Sahara and found very rapid
changes in water chemistry, from fresh to highly saline, within this time interval.
At the site of Adrar Bous in the Tenere Desert of the south-central Sahara, an early
Holocene lake associated with Mesolithic remains dried up and was succeeded by
a shallower lake associated with Neolithic artefacts, food remains, human burials
and the complete skeleton of a domesticated short-horned cow (
Bos brachyceros
).
Gastropod shells from the Neolithic lake and charcoal from the Neolithic sites had
radiocarbon ages between approximately 6 ka and approximately 4.5 ka (Williams
et al.,
1987
; Williams,
2008
). Analysis of the stable carbon and oxygen isotopic com-
position of the shells indicated some degree of seasonal variability and possibly cooler
temperatures but certainly wetter and less evaporative conditions at that time. Another
important result was the differential response shown by several different species of
gastropod, indicating the need for caution in this type of research.
Ayliffe et al. (
1996
) analysed the stable carbon and oxygen isotopic composition of
gastropods that lived in shallow ponds in the present desert west of the lower White
Nile during the early Holocene (Williams et al.,
1974
). They concluded from the
highly negative
18
O values of the shells that the region experienced less evaporation
at that time and inferred from the extreme variability in isotopic composition (up to
6-7
‰
PDB) that there had been considerable interannual variability in precipitation.
