Palaeomagnetic analysis of the Sterkfontein palaeocave deposits: Implications for the age of the hominin fossils and stone tool industries

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

“…As a result of this practice, the uncertainty limits given for the isochron regression of Granger et al 3 as well as the underground production rates derived from it are underestimated by an unknown amount. Our lower values for the in-situ produced 26 Al and 10 Be are likely to fit comfortably within the real uncertainty limits from the isochron regression and are thus not in conflict with these.…”

“…In Point 2, Stratford et al 1 find no fundamental fault in our direct calculation of the underground production rate of 10 Be and 26 Al, but criticise the fact that no uncertainty limits were given, and state (1) that we used a constant erosion rate, (2) that we fixed the 26 Al/ 10 Be production rate at 8.1, (3) that depth, density and erosion rates in the past are not known with sufficient confidence and (4) that production rates at depth cannot be calculated exactly, citing Balco 16 , an excellent review that became available online in February 2017, when our paper was in press. They further state that their isochron regression solves directly for underground production rates, implying that our estimates were unnecessary.…”

“…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. 2 As we describe 2 , the upper limits of production rates for which the 26 Al and 10 Be concentrations in MC2A are realistic coincide with the range of directly calculated values.…”

“…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. 2 As we describe 2 , the upper limits of production rates for which the 26 Al and 10 Be concentrations in MC2A are realistic coincide with the range of directly calculated values. The fact that Stratford et al 1 do not comment on this indicates that they accept it.…”

“…The chert fragments must indeed have been derived from a few metres below surface, as indicated by their aggregate 10 Be and 26 Al concentrations. That is all we know for certain.…”