JACOBS, BONNIE F.1* and PATRICK S. HERENDEEN2. 1Environmental Science Program, Southern Methodist University, P.O. Box 750395, Dallas, TX 75275-0395; 2Department of Biological Sciences, The George Washington University, 2023 G Street NW, Washington, D.C. 20052. - Evidence for dry climate at the Eocene Mahenge site, north-central Tanzania.
The Mahenge site consists of crater lake deposits formed subsequent to
a kimberlite eruption dated at approximately 46 Ma. The sediments
contain plant fossils dominated by leaf remains, with a smaller number
of seeds and fruits, as well as abundant fish remains, and other
vertebrates including a bat and frog. This locality provides a unique
opportunity to evaluate an environment at a tropical low latitude in
the Middle Eocene, a time of warmth at high latitudes and unresolved
global circulation patterns. A minimum of 20 species (based on leaf
morphotypes) is present among the approximately 200 plant specimens,
which include at least 7 species in the Leguminosae family. Among
these is the genus Acacia, which today is limited to hot or warm,
seasonally dry environments. A preliminary analysis of leaf area,
which is positively correlated with mean annual and wet season
precipitation, indicates that rainfall at Mahenge was approximately
700 mm/year, or about 400mm/yr less than what would be required to
support tropical forest vegetation today. The Mahenge flora provides a
first data point for paleoclimate in the Eocene of tropical Africa and
the paleovegetation reconstruction fits well within the larger
framework established by a limited number of palynological and fossil
wood records from North and West Africa. These have been interpreted
as indicating relatively low diversity communities in the Early to
Middle Eocene, followed by the initial development of lowland forest
represented by increasing diversity in the Late Eocene and Oligocene.
Our interpretation ofthe Mahenge paleocommunity is concordant with the
hypothesis that forest vegetation was not yet established within the
tropical belt of Africa 46 million years ago.
Key words: Africa, African paleobotany, Eocene, Tanzania paleoclimate