Geoscience Reference
In-Depth Information
magnitude of the radiocarbon reservoir effect, which varied between
−
1,200 and a
10,700
14
C years, indicating that uncorrected ages would appear
far older than they actually were. They found that the humid phase began between
13,000 and 12,000
14
C yr BP, with maximum lake levels attained between 10,800 and
9,200
14
C yr BP. Their results do not accord with those of Sylvestre et al. (
1999
)
discussed in the previous paragraph in regard to the timing of the wettest phase. One
possible explanation for the difference between the two sets of ages bracketing the
wettest phase is that the lakes in the two studied areas were influenced by air masses
derived from quite different sources operating at different times.
The evidence afforded by desert lakes in South America to help reconstruct past
hydrological and environmental changes is a very useful first step towards establishing
a history of past climatic changes in presently arid areas. However, the light cast by
Geyh et al. (
1999
) and by Sylvestre et al. (
1999
) on the magnitude of the radiocarbon
reservoir effect and its changes over time causes us to ask whether the frequency
analysis of radiocarbon ages for lake level fluctuations in the deserts of North America
conducted by Smith and Street-Perrott (
1983
) may have overestimated the radiocarbon
ages of some of the high lake levels in that region. Needless to say, where appropriate
steps have been taken to determine this effect over time in a particular lake basin,
there should be no cause for concern.
surprisingly large
−
12.5 East African pluvials
J.W. Gregory was among the first to map the geology of the East African Rift Valley.
In the course of his fieldwork in southern Kenya in 1893, he found evidence of high
lake levels in the form of thick deposits of diatomite on the Kamasian escarpment west
of Lake Baringo. Although he considered them to be Miocene in age (Gregory,
1896
),
L.S.B. Leakey later discovered Early Stone Age Acheulian hand-axes and cleavers
associated with these lake deposits, indicating a Pleistocene age (Leakey,
1931
). At
about the same time, E.J. Wayland, who was then Director of the Geological Survey
of Uganda and keenly interested in African archaeology, also noted the presence of
previously more extensive lakes and wetlands, and in 1930 he discovered an important
prehistoric site on the Kagera River near the Kavirondo Gulf of Lake Victoria. Leakey,
who had already embarked on a systematic survey of the Stone Age cultures of Kenya
in 1926 (see Leakey,
1931
), was quick to recognise that many of his prehistoric sites
appeared to be in places that were once much wetter than they are today. Assisted from
time to time by Wayland and later by the Swedish geologist Nilsson (
1931
;
1935
;
1940
;
1949
), Leakey devised a climatic chronology based on supposed pluvials, named
after the type localities where they were considered to have occurred. The oldest, or
Kageran
, was considered to be early Pleistocene in age and to be the possible time
equivalent of one of the earlier Alpine glaciations, perhaps the Gunz. The
Kamasian
pluvial he equated with the Alpine Mindel glaciation; followed by the
Kanjeran
and
