Into the Rift: The Geology of Human Origins in Eastern Africa
Anatoly N. Zaitsev, Charles Musiba, and Lindsay J. McHenry – Guest Editors
Table of Contents
Overview
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2023 Topics
Overview
Spanning from the horn of Africa down to Lake Malawi, the East Africa Rift preserves a plethora of paleoanthropological sites (e.g., Olduvai Gorge, Turkana, Awash) that document our evolutionary journey spanning the last seven million years of Earth’s history. A common feature of these sites is that they are associated with volcanic-sedimentary basins intimately related to the development of the rift. Radiometric and paleomagnetic geochronology helps establish age relations between different hominin species. Geochemical studies of primary tuffs help correlate between sites and identify specific volcanic sources. Authigenic minerals formed in rift lakes can help reconstruct past climates and environments. Recent excavations at Laetoli show that this valuable site is slowly disappearing owing to diagenetic processes, and preservation of the footprints is essential and urgently needed.
- The Dawn of Humanity: What Can Paleoanthropologists and Geoscientists Learn from One Another?
- Tectonic and Paleoclimatic Setting for Hominin Evolution in Eastern Africa
- Using Radiometric Dating, Magnetostratigraphy, and Tephrostratigraphy to Calibrate Rates of Hominin Evolution in the East African Rift
- Paleolakes of Eastern Africa: Zeolites, Clay Minerals, and Climate
- Laetoli: The Oldest Known Hominin Footprints in Volcanic Ash
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v19n3 Olivine
Guest Editors: Emily C. First (Macalester College, USA), Philipp Ruprecht (University of Nevada-Reno, USA), and Benoît Welsch (Macalester College, USA) To study the Earth and other planetary bodies, one has to understand the mineral olivine; when and where it forms, how it crystallizes and deforms, and how it responds to changing environmental conditions in the Earth’s interior, on the surface, and in space. This issue will take a look at olivine research from the atomic scale to the Solar System and beyond. Olivine plays a critical role for plate tectonics, magma transport from mantle sources to the surface, how the Solar System differentiated and evolved, and also for the efforts addressing climate change through carbon capture and storage. This issue will highlight olivine research that crosses many disciplines, from seismology and geodynamics, petrology and volcanology to low-temperature geochemistry and remote sensing.- Olivine – The Little Green Science Machine Benoît Welsch (Macalester College, USA), Emily C. First (Macalester College, USA), Philipp Ruprecht (University of Nevada-Reno, USA), and Michael C. Jollands (Gemological Institute of America, USA)
- Hide and Seek: Trace Element Incorporation and Diffusion in Olivine Michael C. Jollands (Gemological Institute of America, USA), Ralf Dohmen (Ruhr Universität Bochum, Germany), and José Alberto Padrón-Navarta (CSIC-Universidad de Granada, Spain)
- Deforming the Upper Mantle: Olivine Mechanical Properties and Anisotropy Sylvie Demouchy (Université Clermont Auvergne, France), Qin Wang (Nanjing University, China), and Andréa Tommasi (Université de Montpellier, France)
- Olivine Exit Interviews—Piecing Together Magmatic Puzzles Philipp Ruprecht (University of Nevada-Reno, USA) and Benoît Welsch (Macalester College, USA)
- Olivine—The Alteration Rock Star Oliver Plümper (Utrecht University, Netherlands) and Juerg Matter (National Oceanography Centre Southampton, UK)
- Galaxy of Green Emily C. First (Macalester College, USA), Christopher Kremer (Stony Brook University, USA), Myriam Telus (University of California-Santa Cruz, USA), and David Trang (University of Hawaiʻi at Mānoa, USA)
2023 Topics
- Alkaline Lakes (February 2023)
- Into the Rift: The Geology of Human Origins in Eastern Africa (April 2023)
- Olivine (June 2023)
- Biomagnetism (August 2023)
- Large Igneous Provinces: Versatile Drivers of Global Change (October 2023)
- Geometallurgy: Resource Optimization for a Sustainable Future (December 2023)
Thematic Articles
Establishing a scientific narrative of human origins requires a better understanding of the geological processes that facilitated the fossil- ization and recovery of hominins and associated fauna that inform us about our human ancestors’ past environments. Paleoanthropologists rely on geologists, particularly volcanologists, geochemists, sedimentologists, and geochronologists, to help them tease out the depositional and preservation history of fossils. Here, we provide an overview of how geology has contributed to major paleoanthropological discoveries from select Plio–Pleistocene locali- ties in eastern Africa, Tanzania (Oldupai* (Olduvai) Gorge and Laetoli), Kenya (Allia Bay, Kanapoi, and Koobi Fora), and Ethiopia (Hadar, Woranso-Mille, and Dikika) over the past 75 years of research.
As dynamic processes in the Earth’s mantle stretch and thin large sectors of the African plate, broad plateaus interrupted by deep valleys and flanking mountains have formed at the Earth’s surface. These vertical and horizontal crustal movements occur concurrent with global climate changes, both of which happen over diverse spatial and temporal scales. Together, they modulated eastern Africa’s habitats for early hominins, and for flora and fauna in general. The habitat for hominin evolution, therefore, is shaped by bottom- up and top-down processes. Broad plateau uplift in Ethiopia had initiated by 30 Ma, coincident with or after flood magmatism at 45 Ma when dry seasonal woodland environments initiated in eastern Africa. The fossil-rich sedimentary sequences partially filling the 30–70-km-wide rift basins record the history of human evolution, as well as the complex interplay between climate change, uplift, volcanism, and faulting in equatorial Africa. The lake shorelines and hydrothermal systems served as oases for hunter-gatherers, and the rough topography of the faulted landscape may have served as refugia. Here, we outline the relevant time–space patterns to establish the geodynamic and paleoclimatic context for human evolution in eastern Africa.
Age-calibration of hominin fossils and artifacts in the East African Rift is principally achieved through dating of associated volcanic-sedimentary strata. The dominant dating techniques for sites >100 ka are the 40Ar/39Ar radiometric dating method, magnetostratigraphy, and tephrostratigraphy. The 40Ar/39Ar technique relies on the occurrence of volcanic deposits in the target strata, which are often present as a consequence of the interplay of rift formation and volcanic activity. The frequency of datable material may be limited; however, by also applying the relative dating methods of magnetostratigraphy and tephrostratigraphy to these same strata, a chronostratigraphic framework can be built and applied to fossiliferous strata. This chapter provides examples of the application of these techniques at Olduvai Gorge, Tanzania, and two areas of the East African Rift in Ethiopia.
The eastern branch of the East African Rift System hosts many shallow modern lakes and paleolakes, which can be sensitive recorders of changing climate conditions (complicated by tectonics) during the past few million years. However, many of such lakes are saline–alkaline (salty and high pH), and these conditions do not easily preserve pollen and other biologi- cally derived paleoclimate indicators. Fortunately, some preserved minerals that formed in these extreme environments reflect subtle shifts in lake water chemistry (controlled by changes in climate conditions) and therefore provide a continuous record of local and regional climate change. We present two different mineral proxies (zeolites and clays) from two different paleolake basins (Olduvai Gorge, Tanzania, and Chew Bahir, Ethiopia) as examples.
Hominin footprints are rare in eastern Africa and known from the Laetoli (Tanzania), Ileret (Kenya), and Melka Kunture (Ethiopia) areas. The prints were made by Australopithecus afarensis, Homo erectus,
and Homo heidelbergensis. Their study is an important source of information regarding hominin body size, anatomy, positional behavior, and locomotion biomechanics. The most-known and best-studied examples are the 3.66-Ma Australopithecus afarensis footprint trackways at Laetoli, which represent the oldest known record of hominin bipedalism in Africa. The footprints occur in a volcanic tuff sequence, which was originally deposited as melilite nephelinite ash. Recent excavations show that this valuable paleoanthropological site is slowly disappearing as a result of surface diagenetic processes. Preservation of the footprints is essential and urgently needed.
