Integrative geochronology calibrates the Middle and Late Stone Ages of Ethiopia's Afar Rift

Abstract

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.

Keywords: Ethiopia; Late Stone Age; Middle Awash; Middle Stone Age; geochronology.

Fig. 1.

Satellite imagery of the northern sector of the Middle Awash study area at synoptic and precision scales. Base image is a three-dimensional perspective view to the east from above the western margin of the Afar Rift. White lettered circles mark the lettered sections in Fig. 2. Inset imagery is panchromatic and multispectral Worldview-2 imagery captured normal to the Faro Daba landscape. Yellow arrows mark the courses of sinuous Pleistocene paleochannels currently topographically inverting due to local erosion. Fossiliferous ∼100-ka Faro Daba beds were emplaced north and east of these channels directly atop darker surfaces of the DMCC that had been locally reexposed by paleo-erosion that removed the older Chai Baro beds. The ∼158-ka DGST of these beds is the most reflective sediment in the lower right corner of the insets. See SI Appendix for details and illustrative ground-truth photographs.

Fig. 2.

Halibee member litho- and chronostratigraphy. Measured sections representative of the OUD (A), Wallia (B–D), Faro Daba (E–H), and Chai Baro (I–M) beds are illustrated, with letters corresponding to locations plotted in Fig. 1. A central schematic composite section summarizes the relationships between these beds. Important marker horizons and the stratigraphic levels of chronologic results and archaeological and paleontological resources are indicated. Photographs of outcrops on which these sections are based are presented in SI Appendix. Descriptions and discussions of the materials and methods used for geochronologic determinations and of the tephra and their chemical signatures are contained in the main text and SI Appendix. A more detailed presentation of the age determinations is summarized in Fig. 3.

Fig. 3.

Radioisotopic ages from the Halibee member; age errors are 2σ or 95% CI. Schematic column (Center) shows relative positions of dated samples. (A) ¹⁴C ages of five OES fragments from locality OUD-A1 in the OUD beds. (B) ¹⁴C and ²³⁰Th/U ages of splits from an OES fragment from locality HAL-A27 in the Wallia beds. (C) ²³⁰Th/U ages of three OES fragments from sample MA15-09 in the Faro Daba beds; plots for each fragment show measured ages for outer- and inner-shell fractions and resulting burial ages (see SI Appendix, section 2.2 for details); remaining plot shows burial ages of each OES fragment and their weighted mean age. (D) ⁴⁰Ar/³⁹Ar data and ages for the DGST. (Left) K/Ca ratios (Upper), ⁴⁰Ar/³⁹Ar ages (Middle), and relative probability distributions of ages of two size fractions of single anorthoclase grains from sample MA15-07 (Lower); shaded areas in middle-left panel indicate grains inferred to be xenocrysts and omitted from the inverse isochron shown in the right panel; note higher xenocryst abundance in coarser grain size. (Right) Inverse isochron for anorthoclase grains from MA15-07 after excluding xenocrysts; the final weighted mean age of the DGST (inset age) includes both data from MA15-07 (this study) and from MA09-04 (11). For details, see SI Appendix and Dataset S1.