Abstract

 
 
 Following the publication (Granger DE et al., Nature 2015;522:85–88) of an 26Al/10Be burial isochron age of 3.67±0.16 Ma for the sediments encasing hominin fossil StW573 (‘Little Foot’), we consider data on chert samples presented in that publication to explore alternative age interpretations. 10Be and 26Al concentrations determined on individual chert fragments within the sediments were calculated back in time, and data from one of these fragments point to a maximum age of 2.8 Ma for the sediment package and therefore also for the fossil. An alternative hypothesis is explored, which involves re-deposition and mixing of sediment that had previously collected over time in an upper chamber, which has since been eroded. We show that it is possible for such a scenario to yield ultimately an isochron indicating an apparent age much older than the depositional age of the sediments around the fossil. A possible scenario for deposition of StW573 in Member 2 would involve the formation of an opening between the Silberberg Grotto and an upper chamber. Not only could such an opening have acted as a death trap, but it could also have disturbed the sedimentological balance in the cave, allowing unconsolidated sediment to be washed into the Silberberg Grotto. This two-staged burial model would thus allow a younger age for the fossil, consistent with the sedimentology of the deposit. This alternative age is also not in contradiction to available faunal and palaeomagnetic data.
 
 
 
 
 Significance: 
 
 
 
 Data on chert samples taken close to StW573 impose a maximum age for the fossil of 2.8 Ma – younger than the 3.67 Ma originally reported. We propose and explore a two-stage burial scenario to resolve the inconsistency and to reopen the discussion on the age of fossil StW573.
 
 
 
Highlights
In a recent contribution, Granger et al.[1] present 10Be and 26Al data on quartz from Member 2 sediments in the Silberberg Grotto in Sterkfontein Cave, South Africa, encasing StW573 (‘Little Foot’)[2], a complete skeleton referred to as Australopithecus prometheus[1]
An altitude correction (1500 m) was applied to the component of cosmogenic nuclides produced by stopped negative muons
While we have shown that the isochron of Granger et al.[1] can be com patible with a two-stage burial scenario, the question remains as to how fossil StW573 could be younger than 2.8 million years old and be embedded in sediments that have been underground for over 3.5 Ma
Summary
Granger et al.[1] present 10Be and 26Al data on quartz from Member 2 sediments in the Silberberg Grotto in Sterkfontein Cave, South Africa, encasing StW573 (‘Little Foot’)[2], a complete skeleton referred to as Australopithecus prometheus[1]. STM2-light is a composite sample, its average 26Al and 10Be concentrations at 2.8 Ma provide the best estimate of what in-situ accumulated cosmogenic nuclide abundances in such a previous higher level cave system could have been; at 2.8 Ma they plot just below the surface production curve (Figure 3). All apparent minimum pre-burial erosion rates are much lower than the erosion rate measured for today using sample ST7.1 This difference may reflect either lower true erosion rates[21,28] or higher chemical erosion factors[29] in the past, with more of the dolomite being removed by dissolution at the surface as a consequence of a more humid climate[30] This analysis demonstrates that an apparent isochron age of 3.67±0.16 Ma can be obtained for a secondary deposit which was laid down at a much younger age (2.8 Ma in our example), but which reworked surface-derived material that had accumulated in an upper chamber over a period as long as 1 Ma (2.8–3.8 Ma) or possibly even longer.
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