Geoscience Reference
In-Depth Information
Figure 14.2. Algal limestone pillars formed when Lake Abhe was full during the late
Pleistocene and early Holocene, Afar Desert, Ethiopia. (Photo: Fran¸oise Gasse.)
in annually banded modern and mid-Holocene travertines/tufas in the Grand Canyon
of Arizona suggests that they were formed as a result of monsoon-generated summer
floods during the mid-Holocene, just as they are today (O'Brien et al.,
2006
).
Brook et al. (
1997
) compared the radiocarbon and U-series ages obtained from
speleothems and tufas from the summer rainfall zone of southern Africa (Namibia,
Botswana, northern Cape and the Transvaal) with those obtained on similar deposits
across Somalia. They found that during the last 250 ka, when conditions were wet in
southernAfrica theywere dry in Somalia, and conversely. In Somalia, speleothem, tufa
and rock shelter sediments indicated wetter conditions in this arid region at 260-250,
176-160, 116-113, 87-75, 13, 10, 7.5 and 1.5 ka. In southern Africa, conditions were
