From the Cover: Integrative geochronology calibrates the Middle and Late Stone Ages of Ethiopia’s Afar Rift

The Halibee member of the Upper Dawaitoli Formation of Ethiopia’s Middle Awash study area features a wealth of Middle and Later Stone Age (MSA and LSA) paleoanthropological resources in a succession of Pleistocene sediments. We introduce these artifacts and fossils, and determine their chronostratigraphic placement via a combination of established radioisotopic methods and a recently developed dating method applied to ostrich eggshell (OES). We apply the recently developed ²³⁰Th/U burial dating of OES to bridge the temporal gap between radiocarbon (¹⁴C) and ⁴⁰Ar/³⁹Ar ages for the MSA and provide ¹⁴C ages to constrain the younger LSA archaeology and fauna to ∼24 to 21.4 ka. Paired ¹⁴C and ²³⁰Th/U burial ages of OES agree at ∼31 ka for an older LSA locality, validating the newer method, and in turn supporting its application to stratigraphically underlying MSA occurrences previously constrained only by a maximum ⁴⁰Ar/³⁹Ar age. Associated fauna, flora, and Homo sapiens fossils are thereby now fixed between 106 ± 20 ka and 96.4 ± 1.6 ka (all errors 2σ). Additional ⁴⁰Ar/³⁹ results on an underlying tuff refine its age to 158.1 ± 11.0 ka, providing a more precise minimum age for MSA lithic artifacts, fauna, and H. sapiens fossils recovered ∼9 m below it. These results demonstrate how chronological control can be obtained in tectonically active and stratigraphically complex settings to precisely calibrate crucial evidence of technological, environmental, and evolutionary changes during the African Middle and Late Pleistocene.

Accurately dating the emergence of Homo sapiens and associated technologies in Africa is an enduring challenge in geochronology and a persistent source of frustration for paleoanthropologists (1–3). Most of Africa’s Middle Stone Age (MSA) lies beyond the ∼50-ka range of ¹⁴C dating, and even Later Stone Age (LSA) occurrences often lack associated charcoal or bone suitable for this technique. Furthermore, crucial Eurasian finds originally dated by ¹⁴C have recently required large revisions, highlighting the importance of improved sample preparation even for well-established dating methods (4–6). The ⁴⁰Ar/³⁹Ar dating of potassium-rich Pleistocene volcanic minerals has yielded solid calibrations based on association and correlation (7–12). However, even when present, many such rocks are contaminated by detrital minerals or lack datable fractions, further limiting application of the technique.Efforts to overcome these geochronological barriers have often led to adoption of less-reliable techniques such as electron spin resonance, thermoluminescence, or other trapped-charge techniques to obtain varying age estimates for important fossils of emergent H. sapiens (refs. 1 and 2, ref. 13 contra ref. 14; ref. 15 contra ref. 16). Such approaches, often pursued when more tested and cross-validated methods are unavailable, require in situ measurements, may involve questionable assumptions, and/or yield less-precise ages than many radioisotopic decay-based methods (1, 17). Even for more established techniques, methodological developments continue to require revision of earlier chronologies (e.g., ref. 18).Here we integrate the results of detailed stratigraphic and geomorphological field studies with satellite imagery, tephra chemistry, and multiple radioisotopic chronometers including the recently developed ²³⁰Th/U burial dating of ostrich eggshell (OES) (19) to calibrate a suite of stratigraphically superimposed fossil and artifact assemblages from the Middle Awash study area, Afar Rift, Ethiopia. This provides the temporal scale against which ongoing and future studies of key fossils and artifacts will be measured and provides a model approach for other occurrences with similar geochronological challenges and resources. We predict that further applications of this integrated, basinal-to-submeter geological approach employing multiple geochronological methods will meet ongoing challenges (20) of calibrating and understanding Pleistocene climate change, tectonic processes, environments, technologies, and biological evolution.The chronostratigraphic framework presented here is foundational to ongoing paleoanthropological studies on a spatially and stratigraphically extensive set of MSA and LSA occurrences in the Dawaitoli Formation of the Middle Awash. We first sketch the history of earlier work and outline our broader stratigraphic findings. We then describe four paleoanthropologically important Halibee member beds. We describe this succession in chronological order, oldest first, summarizing geological and geochronological results and introducing the paleoanthropological content of each. The broader implications of this research are then considered.