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

Ellen D. Currano; Bonnie F. Jacobs; Rosemary T. Bush; Alice Novello; Mulugeta Feseha; Friðgeir Grímsson; Francesca A. McInerney; Lauren A. Michel; Aaron D. Pan; Samuel R. Phelps; Pratigya Polissar; Caroline A.E. Strömberg; Neil J. Tabor

doi:10.1016/j.palaeo.2019.109425

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

Ellen D. Currano^*, Bonnie F. Jacobs, Rosemary T. Bush, Alice Novello, Mulugeta Feseha, Friðgeir Grímsson, Francesca A. McInerney, Lauren A. Michel, Aaron D. Pan, Samuel R. Phelps, Pratigya Polissar, Caroline A.E. Strömberg, Neil J. Tabor

^*Corresponding author for this work

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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.

Original language	English (US)
Article number	109425
Journal	Palaeogeography, Palaeoclimatology, Palaeoecology
Volume	539
DOIs	https://doi.org/10.1016/j.palaeo.2019.109425
State	Published - Feb 1 2020

Funding

We thank the Authority for Research and Conservation of Cultural Heritage for permission to conduct research in the Mush Valley and the director and staff of the National Museum of Ethiopia, particularly T. Getachew and Y. Assefa, for facilitating the paleobotanical research presented here. We are grateful to the people of Upper and Lower Mush for their hospitality, and to M. Clemens, L. Jacobs, J. Noret, and T. Tesfamichael for field assistance and discussion. Three anonymous reviewers provided constructive comments on our manuscript. Our work was funded by U.S. National Science Foundation grants EAR 1053549 (BFJ), EAR 1052478 (EDC), EAR 1053351 (FAM), and EAR 1253713 (CAES), National Geographic Society CRE 8816-10 (EDC), Marie Curie-Sklodowska IOF 659596 “MACEA” (AN), the Austrian Science Fund, FWF , P29501-B25 (FG), and the Australian Research Council FT110100793 (FAM).

Keywords

Africa
Mush Valley
Paleobotany
Paleoenvironmental proxies
Phytoliths
n-Alkanes

ASJC Scopus subject areas

Oceanography
Ecology, Evolution, Behavior and Systematics
Earth-Surface Processes
Palaeontology

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10.1016/j.palaeo.2019.109425

Cite this

Currano, E. D., Jacobs, B. F., Bush, R. T., Novello, A., Feseha, M., Grímsson, F., McInerney, F. A., Michel, L. A., Pan, A. D., Phelps, S. R., Polissar, P., Strömberg, C. A. E., & Tabor, N. J. (2020). Ecological dynamic equilibrium in an early Miocene (21.73 Ma) forest, Ethiopia. Palaeogeography, Palaeoclimatology, Palaeoecology, 539, Article 109425. https://doi.org/10.1016/j.palaeo.2019.109425

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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.",

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AU - Feseha, Mulugeta

AU - Grímsson, Friðgeir

AU - McInerney, Francesca A.

AU - Michel, Lauren A.

AU - Pan, Aaron D.

AU - Phelps, Samuel R.

AU - Polissar, Pratigya

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AU - Tabor, Neil J.

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N2 - 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.

AB - 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.

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

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

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Data for: Ecological dynamic equilibrium in an early Miocene (21.7 Ma) forest, Ethiopia

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