If I were a chess player I probably wouldn’t be trying this gambit. But I’m not. So, bear with me.
|Worked bone, stone and ochre from Blombos Cave (Wikipedia)|
I need help with a question that’s been nagging me for years. I’m curious to know if any of the bone artifacts from the Middle Stone Age (MSA) layers at Blombos Cave have been directly dated using AMS 14C. From my reading it appears not. In fact, it looks as if 14C was abandoned in favour of luminescence techniques once they had excavated deeper than those layers identified as Late Stone Age, the earliest of which were dated to give-or-take 39 ka BP. Everything below that is deemed to be MSA, and organic materials such as charcoal and bone were passed over in favour of grains of sand or burned flints in those strata.
Remember that, for most of us, MSA is synonymous with the Middle Palaeolithic in the rest of the world, and, for better or worse, it’s exclusively associated with the Neanderthals and their ilk, for which the jury is still out as to their cognitive equivalency with us modern types. Finding what are clearly modern human artifacts at Blombas Cave and elsewhere on the order of 30 to 50 ka earlier than anywhere else in the world has stunned and amazed scientists from Barrow to Burbank. But it’s never sat well with me.
|Location of Blombos Cave, South Africa (Credit)|
You and I know that 14C is perfectly capable of accurately gauging the age of organic materials until at least 50 ka, notwithstanding the need for calibration that corrects for environmental and other effects. Why then do we have only luminescence age determinations for the sediments in which the Blombos Cave bone artifacts and charcoal were deposited? Henshilwood et al. (2002) provide some insight, although I’m not certain they realize that by doing so they’ve left themselves exposed to (at a minimum) questions about their decision.
That (to me) curious decision is explained in what amounts to a throwaway comment, which I will quote here
In radiocarbon terms, the MSA at BBC is of infinite age (Vogel, personal communication).
The MSA levels are being dated using luminescence techniques: single-grain laser luminescence (SGLL), single aliquot optically stimulated luminescence (OSL and IRSL), multiple aliquot OSL on sediments and also TL of burnt lithics and electron spin resonance (ESR) of teeth (Henshilwood et al. 2002:638).
Being the skeptical type, I was intrigued by what seemed to me to be such a weak citation as to the inefficacy of 14C beyond 39 ka–the date of the oldest LSA at Blombos. Just a ‘Vogel pers. comm.’ No reams of empirical evidence. No other justification. So I endeavoured to discover by what authority Vogel had made such a pronouncement.
No doubt some among you will think it naïve of me, or worse, that I’m poorly informed and ill-prepared to be taken seriously by the palaeoanthropological establishment. Nevertheless, I had no prior knowledge of Vogel’s reputation in the radiocarbon world and in the South African archaeological community. His work includes a 1997 paper in Radiocarbon in which he attempted to calibrate 14C dates with U/Th dates within the same stalagmite from a South African cave. That work was superseded a few years later by the more widely cited Fairbanks et al. (2005), who honed the 14C calibration curve back to 50 ka using pristine corals from around the globe. Their findings are that 14C underestimates calendar years such that 45 RCYBP works out to 48,934 calendar years (give or take 500). By this means the calendar date of 39,200 BP from the lowest LSA level at Blombos would have been produced by a radiocarbon age of about 34 ka RCYBP (try it yourself by clicking here to go to Fairbanks’s calibration calculator).
It would seem, therefore, that despite Vogel’s pronouncement, cited in Henshilwood et al. (2002), there is no physical limitation on dating organic material that is older than 39 ka (i.e 34,000 RCYBP), as long as its age doesn’t exceed 45 RCYBP. That would, theoretically, allow the excavator of Blombos Cave to extend use of 14C for at least a further 11,000 RCY beyond the earliest LSA from Blombos Cave. Surely some of the MSA materials could be presumed to date from this 11,000-year window.
Directly dating the bone and charcoal from the upper MSA strata may prove nothing. However, knowing that there’s no theoretical limit on the use of 14C for that 11,000 year period leaves wide open the question as to why Chris Henshilwood hasn’t attempted to date, directly, some of his bone artifacts or charcoal from the MSA layers. It’s just possible that they would yield dates far younger than those produced by luminescence techniques (about which I’ve had a certain amount to say in previous efforts here at the Subversive Archaeologist).
|From Henshilwood et al. (2002)|
After all, the MSA levels in Blombos (at least those illustrated in Henshilwood et al. , shown above) appear to be quite shallow–the sort of depth that you could imagine accumulating in far less than the 35 ka that the luminescence dates would have you believe it took for them to accumulate–and it’s within reason to suspect that they could easily have been deposited in the 11 RCY or so before 39,200 BP.
And so. For what it’s worth, I’m issuing a challenge from this lofty perch of mine. Chris, try dating some of your MSA bone using good, old-fashioned, AMS 14C and see what you get. A fair few of us are curious to know the result.