Multiple Proxies Provide Paleoecology, Paleoclimate, and Paleolandscape Reconstructions for the Late Oligocene of Northwestern Ethiopia

Late Oligocene (28 – 27 Ma) strata from northwest Ethiopia provide a unique opportunity to reconstruct Paleogene tropical African paleoecology using evidence from numerous proxies. Geochemical, sedimentological, and paleontological studies of strata from Chilga Woreda are yielding data that provide an increasingly detailed understanding of the ancient landscape, paleoclimate, and biotic communities. The taxonomic compositions of parautochthonous and autochthonous leaf compression and wood assemblages document forest communities dominated by trees with affinities to relatives found today in Central and West Africa or the Eastern Arc and coastal forests of East Africa. These forests differ from their modern counterparts by the common occurrence of palms, which today are rare or absent. Based upon trunk diameters, in situ silicified trees indicate canopy height from 24- 30 m. Ash beds document more transient pioneer communities dominated by ferns, legumes, and palms. The associated faunal remains include large mammals representing the last occurrences of some archaic African endemics such as arsinoitheres, which are found with the earliest occurrences of other groups such as deinothere proboscideans.

Abiotic parts of the paleoecosystem are documented by geologic mapping and sedimentological studies, which indicate the landscape was gently undulating, and had a spatially and temporally varying high water table. Paleoclimate estimates are derived independently from oxygen isotopes in paleosol phyllosilicates and siderites, chemical analyses of paleosols, the overlapping distributions of modern relatives of fossil plants, foliar physiognomy, and the cellular anatomy of fossil woods. All these methods provide statistically overlapping estimates, which indicate warmer temperatures and more equable rainfall distribution having fewer dry months than today. ä13C of fossil herbivore tooth enamel and sedimentary organic matter from lignites and carbonized wood indicate these animals were consuming various parts of the ancient flora including elements not yet sampled by us for carbon isotopic analysis.