Category Archives: OSL dating

Hominid Hunting, Pinnacle Point, and Me. An epilogue.

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Hominid Hunting has just covered the latest news from Pinnacle Point, the subject of yesterday’s blurt. I couldn’t let it go without comment. Hell, I think by now the author, Erin Wayman, probably expects it!
     It’s ‘awaiting moderation’ as I write this. However, in case it’s considered too technical or too contrary for publication, I’m gonna quote myself here, verbatim. Here’s how it goes.

Hi, Erin. Pinnacle Point is quite an amazing archaeological locality. And the excavators, Brown and Marean, are doing excellent work. I know that it’ll sound a bit like sour grapes, however, for years now there have been those who question not the finds, but the age estimates of these and other southern African sites. Like those at Pinnacle Point, the spectacular finds derive from caves, where organic preservation is often enhanced either by protection from the elements or the chemistry of the cave environment, or both. Unfortunately, for those parts of the site that are beyond the radiocarbon limit—around 40,000 years—the excavators have relied on less precise dating techniques. One in particular, optically stimulated luminescence (OSL), begins with the assumption that a given grain of quartz sand was exposed to sunlight for a time before final burial in the cave. Where caves are concerned this assumption is an untenable assumption, because no one is able to say ahead of time, with any certainty, that a single grain was or wasn’t exposed to sunlight sufficiently powerful or for long enough. As a result, once the technique has yielded its raw results, there follows a complex mathematical dance based on all sorts of other assumptions, as a means of ‘eliminating’ the uncertainty of whether or not a given quartz grain had been sufficiently exposed to sunlight. Moreover, if the mathematical assumptions and the inherent complexity of the calculations are going to be in error they’ll always overestimate the time since that quartz grain (and the artifacts in proximity that are of interest) was last exposed to sunlight. Logically, there’s no under-estimating ages in caves using OSL. This leaves us with a question, one that I think the excavators of Pinnacle Point and elsewhere cannot logically escape. If the dates of unequivocally modern human behavior elsewhere in the world are less than about 45,000 years old, what are the odds that the anomalously early dates for similar behaviour in those southern African sites—all dated by OSL—are systematically overestimating their age? I’d say those odds were far better than the likelihood that all of the finagling involved in arriving at an OSL age estimate is yielding accurate and precise chronometric results.
[Update 0045 UTC November 10, 2012: Doh! I knew I was forgetting something! The bedrock in which the Pinnacle Point caves are developed is described as quartzitic. Wanna know how long it’s been since the quartz grains in the bedrock last saw sunlight? Your guess is as good as mine. Suffice it to say that it’s unlikely that they had been exposed to very much sunlight once they fell to the floor. All that matters is that the uncertainty introduced by one minimally exposed grain would introduce one hell of a wrench in any age estimate, even one that’s been through the math mill.]  

I think that about sums it up. Now, why couldn’t I have said that yesterday?

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Throwing in the Towel Thanks to Pinnacle Point, South Africa. As If.

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At home. Under the weather. What else to do but watch the news ticker. It is ablaze with the latest news from Pinnacle Point in South Africa. The first word I had of it was a headline about this piece, just published, in Nature. 


In the article, long-time acquaintance and Mousterians ‘R’ Us advocate Sally McBrearty extolls the collection of more evidence for modern human behaviour at around 70,000 years. Sally was commenting on a Letter to Nature also published electronically yesterday. 


As you know from reading the SA, there are plenty of claims out there for modern human cognitive abilities or behaviours, or both, going well beyond 100 ka–blade industry at Kathu Pan 1 and Qesem Cave; hafting with bitumen or birch tar, and so on. However, as you also know, I’ve been critical of a great many of them. 
     Yet, according to the age estimates for unequivocally modern human activities at places in southern Africa, such activities are occurring at an age anywhere from 30 ka to 60 ka earlier than they do in Europe and southwestern Asia. For example, finds of decorated ochre and pigment processing ‘kits’ at Blombos Cave ca. 75 ka. In today’s piece by Brown et al. we’re hearing about use of the bow and arrow ca. 70 ka [I haven’t had the time to examine the actual evidence for the claim other than to note that they refer to the microlithic portion of the assemblage consisting of small backed blades.]. This of course is one of the inferences frequently made for the kind of stone tools comprising the Howieson’s Poort phase of what in Africa is called the MSA (Middle Stone Age). 

PP5-6 excavation. Deeply stratified and sharply inclined, much like a cone of talus [Credit: Erich Fisher; (inset) Simen Oestmo] © Science/AAAS.



     
I’ve previously said quite a bit about these early dates, here and here. However, this latest publication, and an earlier one from the same [or similar] group of authors, published in Science a few years back  claims [and at this point I have no reason to doubt them] that they have evidence of heat treating lithic raw material to make it more workable, between about 70 ka and 164 ka! This would be a great discovery even if a mere 30,000 years old. Unfortunately for me, these two inferences–of bows and arrows and heat treating–leave me with a new emotion–dismay–at the precocity of the ancient southern Africans. That’s because these microliths and heat-treated lithics from Pinnacle Point derive from a part of the Pinnacle Point locality known at PP5-6, which is generally described as a rock shelter.

Pinnacle Point PP5-6. Tarpaulin covers the excavations. © Science/AAAS.

      In my earlier rantings I suggest that using Optically Stimulated Luminescence (OSL) age determinations drawn from buried cave sediments may be the culprit, artificially overestimating the age of these patently modern human assemblages. That’s because there’s really no way of knowing, a priori, whether or not an individual quartz sand grain had ever been exposed to the sun, much less for how long. [This, you’ll remember, is a crucial assumption of the technique.] It’s this uncertainty that causes OSL experts to undertake what appears to me to be a complex series of calculations and extrapolations designed to overcome the potential shortcomings of the technique. However, a rock shelter is a bit stickier. Presumably a rock shelter would be exposed to sunlight year in and year out, and thus you might imagine that any quartz grain that came to rest there had been exposed to sunlight long enough to have its ‘clock’ set to zero [unless, of course, that grain had been stripped from older deposits, redeposited in the rock shelter and immediately covered by sufficient material to preclude its being affected by the sunlight thereafter–like, perhaps, at night]. Now, after about a day and a half of off and on Googling I’ve been able to sight nothing written that describes the geomorphic history of PP5-6. Nevetheless, I have a hypothesis that will allow me at least some peace of mind. Given its proximity to a series of true caves at Pinnacle Point, and given its location at the edge of the escarpment that contains those caves, it seems highly likely that, while exposed today, the sediment accumulation at the site occurred inside a cave in which the roof and shoreward wall have subsequently collapsed and migrated down slope.
   So, unless I’m mistaken, I have no more reason to accept these early dates from the Pinnacle Point locality than I do the others from caves in South Africa. I know that the Very Serious Palaeoanthropologists like Curtis Marean will simply scoff at my scepticsm. Nevertheless, one can’t help but wonder. And one day, perhaps, someone will try to directly date a piece of bone from one of those amazingly old stratigraphic units and Voila!
     I live in hope. No. Scratch that. As John Cleese’s supine character in Clockwise moaned, ‘It’s not the despair…It’s the hope.’

   
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Thanks for dropping by! If you like what you see, follow me on Google Friend Connect or Twitter, friend me on Facebook, check out my publications at Academia, or connect on Linkedin. You can also subscribe to receive new posts by email or RSS [scroll to the top and look on the left]. And please don’t forget. Oh, and you can always put me on your blogroll! By the way, I get a small commission for anything you purchase from Amazon.com if you go there by clicking through from this site.