Cosmogenic nuclide dating of Australopithecus at Sterkfontein, South Africa

Abstract

Sterkfontein is the most prolific single source of Australopithecus fossils, the vast majority of which were recovered from Member 4, a cave breccia now exposed by erosion and weathering at the landscape surface. A few other Australopithecus fossils, including the StW 573 skeleton, come from subterranean deposits [T. C. Partridge et al., Science 300, 607-612 (2003); R. J. Clarke, K. Kuman, J. Hum. Evol. 134, 102634 (2019)]. Here, we report a cosmogenic nuclide isochron burial date of 3.41 ± 0.11 million years (My) within the lower middle part of Member 4, and simple burial dates of 3.49 ± 0.19 My in the upper middle part of Member 4 and 3.61 ± 0.09 My in Jacovec Cavern. Together with a previously published isochron burial date of 3.67 ± 0.16 My for StW 573 [D. E. Granger et al., Nature 522, 85-88 (2015)], these results place nearly the entire Australopithecus assemblage at Sterkfontein in the mid-Pliocene, contemporaneous with Australopithecus afarensis in East Africa. Our ages for the fossil-bearing breccia in Member 4 are considerably older than the previous ages of ca. 2.1 to 2.6 My interpreted from flowstones associated with the same deposit. We show that these previously dated flowstones are stratigraphically intrusive within Member 4 and that they therefore underestimate the true age of the fossils.

Keywords: Australopithecus; Sterkfontein; burial; cosmogenic; karst.

Fig. 1.

Map and cross section of Sterkfontein showing sample locations. (A) Map shows the extent of surface deposits and excavations superposed on the cave system. Sample locations reported here are shown as green circles; selected hominin fossils are shown with red stars and U-Pb-dated samples with yellow circles. Universal Transverse Mercator (UTM) coordinates are shown. (B) Cross section of the surface deposits along east-west red line in A. Cosmogenic sample locations are in green circles, and flowstone sample BH4-9 from ref. in BH 4 is shown as a yellow circle. Measured bedding shows that the flowstone is located stratigraphically between the cosmogenic samples, although like other flowstones in Member 4, it is likely intrusive and younger than the breccia. Cross-section topography based on light detection and ranging (LiDAR) collected at the surface and underground. Borehole 4 stratigraphy is based on ref. .

Fig. 2.

Stratigraphic sections and associated photos showing previously dated flowstone. Two sections are located at red bars shown in the base map found in the figure legend. (A) North-south section shows that the previously dated flowstone OE-14 (5) is not in stratigraphic contact with Member 4 but instead is separated by fins of dolomite and decayed dolomite that were removed by blasting. Its age therefore does not constrain that of Member 4. (B) Detailed section of the OE-14 flowstone (5) shows that it lies on decayed dolomite and reworked decayed dolomite breccia derived internally within the cave. The flowstone is overlain by and interfingers with orange sandy microbreccia with no clear stratigraphic relation to Member 4 or Member 5. The north-south cross section intersects at ca. 3.5 m on the west-northwest–east-southeast section, at the plaque.

Fig. 3.

Member 4 isochron. Best-fit isochron curve with 1σ uncertainty band shaded. Postburial production is determined by the intercept of the isochron with the calculated postburial ²⁶Al/¹⁰Be line. The isochron fits all data to within analytical uncertainty shown as 1σ error ellipses.