Kramers, Jan D., and Dirks, Paul H.G.M. (2017) The age of fossil StW573 ('Little Foot'): an alternative interpretation of (26)AI/Be-10 burial data. South African Journal of Science, 113 (3-4).

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Abstract

Following the publication (Granger DE et al., Nature 2015; 522: 85-88) of an (26)AI/Be-10 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. " 0Be and 2 0 AI 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.

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