Why No 14C Dates for Blombos Cave's MSA? Not Lobbing Aspersions. Just Sayin' ...

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. [2002], 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. 

Alas! It's Not Up To Me: OSL Dating South African Caves

It seems almost foolish to say such things. But, if I ran the world (even if only the archaeological world), there'd be some calling up on the carpet during my reign.
     For example, I'd make sure that people claiming great antiquity for modern human behaviour in southern Africa were called out for using optically stimulated luminescence to date cave sediments. After all. The assumption of OSL is that the mineral grains subjected to stimulation were, at some point, exposed to the direct sunlight for long enough to allow all of the trapped electrons in their crystal lattices to have escaped, and the 'clock' effectively set to zero. Otherwise the OSL age determination is going to be older than the reality by some indeterminate amount. How, I ask, is it possible to know that grains found in a cave where, presumably, direct sunlight is at a premium could ever be presumed to have been exposed long enough? Complex 'correction' algorithms notwithstanding, how do they know? If I were the monarch of all things archaeological they wouldn't be able to report age determinations without serious caveats accompanying them. And, I ask, what's the point if you have to say, 'but they might be wrong?'
     Furthermore, given that, regardless of location, daylight gives way to night at least half the time, how do we ever know that mineral grains were exposed to daylight long enough to 'zero the clock'? Really. Half the lifetime of any given mineral grain has been spent in darkness. What are the odds that any given (especially) sand grain was exposed, eroded, deposited and buried in a single night? Happens all the time in the desert. No? Well, then it can happen to a cave or a rockshelter. And in the case of sandstone rockshelters, how do you ever decide what's a sand grain from somewhere else, or a sand grain that was exfoliated from the bedrock? Pray, tell me. 
     So, if I multiply an error-prone estimate by an error-prone estimate, what do I get? It very well could be an accurate age determination. But who's going to know?
     Help me on this one, folks! There are claims for modern human behaviour beginning upwards of 75 to 100 kya in South African caves and rockshelters. What happened between then and about 45 kya, when Europe and indeed the entirety of Asia including Australasia were sprouting human populations almost simultaneously, where no humans trod before? Tell me. Someone. Please. Why a 30 to 65 kyr lag? Hmmmm?
     I guess you'd have to call than an open question. Well, you might. But I think there's nothing open about it. I think the dates are suspect. Pure and simple. Suspect and then some.