Abstract

KRAMERS, Jan D.  and  DIRKS, Paul H.G.M.. The age of fossil StW573 ('Little Foot'): An alternative interpretation of ²⁶Al/¹⁰Be burial data. S. Afr. j. sci. [online]. 2017, vol.113, n.3-4, pp.1-8. ISSN 1996-7489.  http://dx.doi.org/10.17159/sajs.2017/20160085.

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.

Keywords : dating; cosmogenic nuclides; muons; Australopithecus prometheus; burial history.

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