Ecological dynamic equilibrium in an early Miocene (21.73 Ma) forest, Ethiopia

Abstract

Miocene paleoecology of East Africa has implications for human origins and understanding the vicariant legacy forests found today on either side of the East African Rift. Fossil leaves preserved in 21.73 million year old lacustrine sediments from the Mush Valley, Ethiopia, provide a unique opportunity to investigate forest composition and dominance-diversity patterns at an ecological scale. We classified and analyzed 2427 leaves in total from two to three quarries within each of six stratigraphic levels, spanning 7 m of section; we estimate each quarry census represents one to three centuries, and 50–60 kyrs separate the oldest and youngest levels. Pollen, phytolith, and compound-specific organic geochemical data were also collected in a detailed stratigraphic context to provide independent, integrated lines of evidence for landscape evolution and lacustrine paleoecology of the system that preserves the macrofossils. Forty-nine leaf morphotypes were documented, and Legume 1 dominated all samples. Nonmetric multidimensional scaling, Jaccard similarity analyses, and diversity and evenness indices demonstrate a degree of change comparable to community ecology dynamics, likely illustrating a dynamic stable state in forest vegetation surrounding the lake. Taxonomic assessments of leaves, phytoliths, and pollen are consistent with a closed canopy forest with limited palm diversity. A high abundance of des-A ring triterpenoid molecules (diagenetic products formed by microbial degradation under anoxic conditions) and very negative δ13C values (<−45‰) of several hopanoid compounds point to anoxic conditions at the lake bottom, consistent with exquisite fossil preservation. The proportion of mid-chain n-alkanes is low, signifying relatively few submerged plants, but increases up-section, which signals shallowing of the paleolake. The Mush Valley locality is unique in Africa with regard to its very early Miocene age and the abundance and quality of organic remains. This densely forested landscape in an upland volcanic region of the Ethiopian Plateau showed resilience amid volcanic eruptions and had botanical affinities with species found today in West, Central, and eastern Africa.