New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan

Abstract

The cave infills at Sterkfontein contain one of the richest assemblages of Australopithecus fossils in the world, including the nearly complete skeleton StW 573 ('Little Foot') in its lower section, as well as early stone tools in higher sections. However, the chronology of the site remains controversial owing to the complex history of cave infilling. Much of the existing chronology based on uranium-lead dating and palaeomagnetic stratigraphy has recently been called into question by the recognition that dated flowstones fill cavities formed within previously cemented breccias and therefore do not form a stratigraphic sequence. Earlier dating with cosmogenic nuclides suffered a high degree of uncertainty and has been questioned on grounds of sediment reworking. Here we use isochron burial dating with cosmogenic aluminium-26 and beryllium-10 to show that the breccia containing StW 573 did not undergo significant reworking, and that it was deposited 3.67 ± 0.16 million years ago, far earlier than the 2.2 million year flowstones found within it. The skeleton is thus coeval with early Australopithecus afarensis in eastern Africa. We also date the earliest stone tools at Sterkfontein to 2.18 ± 0.21 million years ago, placing them in the Oldowan at a time similar to that found elsewhere in South Africa at Swartkans and Wonderwerk.

age of fossil StW573 (‘Little Foot’): Reply to comments by Stratford et al. (2017)

“…That is all we know for certain. We do not see an essential difference between our wording and that of Granger et al 3 Stratford et al's 1 assertion here that they have to come from the same chamber is unfounded.…”

“…This suggests any opening in the roof of the Silberberg Grotto was connected to a higher level chamber in the cave, and at the time of deposition of Member 2 was unlikely to connect directly to the surface. We further emphasise again that, following the erosion rate of about 5 m/Ma reported by Granger et al 3 , the land surface at Sterkfontein should have been about 14 m higher at 2.8 Ma than it is today, and thus there was room for a cave chamber above the Silberberg Grotto at that time. The suggestion of a previous upper cave above Silberberg Grotto is thus not entirely speculative, as Stratford et al 1 claim.…”

“…It is for that reason that we re-examined the data in order to find if this reported burial age truly represents the only possible interpretation of these data for yielding the age of the fossil. We found that one aspect of the data, not considered by Granger et al 3 , prohibited an age older than 2.8 Ma for the fossil, and in order to resolve the paradox we proposed a two-stage burial scenario, with the sediment material encasing StW573 having been derived from a higher level cave chamber, rather than directly from the surface. Concluding, we state that, as this scenario 'can reconcile the indicated 2.8 Ma maximum age for the fossil with the much older isochron date, it deserves serious consideration' 2 .…”

“…We did not fix the 26 Al/ 10 Be production rate, but calculated 26 Al and 10 Be separately. We used both a zero erosion model and one with an erosion rate of 5 m/Ma (as found by Granger et al 3 ). The depth of StW573 below the present day surface (23 m) was taken from Clarke 13 although Partridge et al 6 give 25 m, which would yield lower production rates.…”

“…A second approach is based on one of the samples analysed by Granger et al 3 -MC2A -which was considered 'reworked' and not included in the isochron regression. We found that the combination of 26 Al and 10 Be concentrations in this sample are impossible if the in-situ production rates yielded by the isochron regression are assumed.…”