A Very Remote Period Indeed: Paleolithic radiocarbon legerdemain

Science Now: Radiocarbon Daters Tune Up Their Time Machine

Two articles of note on the subject of carbon dating have appeared over the past week, the first of which looks at the way in which in which some online writers – myself included, I’m afraid – don’t always give correct dates when considering radiocarbon years and calendar years, with the result that the order in which events occurred, or how artefacts and remains are placed in their Late Middle and Upper Palaeolithic context, can by default, be misleading. For the sake of space and brevity I’ve included a couple of extracts from both posts, but needless to say, both are worth reading in their entirety.

First we hear from Asst. Professor Julien Riel-Salvatore, who writes thus:

Two fundamental but often underappreciated aspects of radiocarbon dating concern the ages it yields and the fact that these ages need to be calibrated in order to get an age that can be expressed in calendar years. I say these aspects are underappreciated because of the way radiocarbon age determinations are usually reported in news reports and, more rarely, in actual research papers.

First, age determinations. Radiocarbon dating is based on the observation that given 14C in previously living organisms decays at a constant, predictable rate. In this case, half of a sample’s 14C decays in about 5730 years in exponential fashion. This means, that after 5730 years, 1/2 of the 14C of a previously living organism remain, 1/4 remains after 11,460 years, etc. The uncertainty or error range reported for all radiocarbon dates is due to imprecision in counting the radioactive decay of carbon atoms in a sample, and it is a critical component of the date.

What the error range indicates is a 66% chance that the age of a sample falls within the interval it brackets. Double the error range, and the resulting interval is 95% likely to include it. What is important to note is that raw radiocarbon age ranges are centered on the date that is statistically most likely to be the correct one for a dated sample. In that sense, it is somewhat warranted to use the date as a shorthand to discuss how old a sample is.

That is, for a bone point dated to 13,400 +/- 100BP (these dates are expressed as before present, with the present assumed to be 1950), it is technically OK to say that it is a 13,400 year-old point, since that age is the most likely to be correct within the interval defined by the error range.

The problem, however, is that radiocarbon years don’t correspond to calendar years, and usually underestimate the true age range of any given sample. This is because the concentration of atmospheric radiocarbon has not been constant over time. However, this problem can be corrected through the use of calibration curves based on the radiocarbon dating of samples of known age and extrapolated from the discrepancy between the two ages. Samples whose calendar age can be determined include historical artifacts, as well as organic remains that grow or accumulate in yearly increments, such as trees (that accumulated a new grwoth ring yearly) or corals.

This is particularly important when considering the chronology of the late Neanderthals, their disappearance from the fossil record, and the implications that may arise therefrom. However, the fact remains that there is apparently good evidence to suggest that Neanderthals survived even longer than traditionally believed, and I’ll try and re-address some of this in a later post in a more accurate way than I’ve done so far.

Next, we hear from Michael Balter, who begins his post by saying:

It took nearly 30 years and a lot of heated debate, but a team of researchers has finally produced what archaeologists, geologists, and other scientists have long been waiting for: a calibration curve that allows radiocarbon dating to achieve its full potential. The new curve, which now extends back 50,000 years, could help researchers work out key questions in human evolution, such as the effect of climate change on human adaptation and migrations.

The basic principle of radiocarbon dating is fairly simple. Plants and animals absorb trace amounts of radioactive carbon-14 from carbon dioxide (CO2) in the atmosphere while they are alive but stop doing so when they die. The steady decay of carbon-14 from archaeological and geological samples ticks away like a clock, and the amount of radioactive carbon left in the sample gives a reproducible indication of how old it is. Most experts consider the technical limit of radiocarbon dating to be about 50,000 years, after which there is too little carbon-14 left to measure accurately.

And follows on by relating:

More recently, however, thanks to new and more accurate data from foraminifers, corals, and other sources–plus some fancy statistical treatments that help predict which way data gaps bend the curve–the INTCAL group has been able to resolve most of the discrepancies. “It took the group quite a while to come together and agree,” says INTCAL team leader Paula Reimer, a geochronologist at Queen’s University Belfast in Northern Ireland.

But the new data, combined with what Reimer calls a “real sense of necessity” among team members to resolve the debates, won the day.  The new curve, called INTCAL09 and published this week in the journal Radiocarbon, not only extends radiocarbon calibration to 50,000 years ago but also considerably improves the earlier parts of the curve, researchers say.  Getting those dates right is critical to understanding such questions as whether humans began painting caves when the climate was colder or warmer, says Clive Gamble, an archaeologist at the University of London, Royal Holloway.

For example, the raw radiocarbon dates for the spectacular paintings of horses, lions, bison, and other animals at Chauvet Cave in southern France, the oldest known cave art, come out at 32,000 years ago, right after a major cold spell hit Europe; but the new calibration curve makes the earliest paintings at Chauvet 36,500 years old, a period of relative warmth.

So the paintings at Chauvet appear to be much older than originally predicted, extending the era of cave painting by around 4 millennia, and by a handy coincidence, there is another post at AVRPI which looks at the dating of the decorated fragments found at Fumane Cave, previously dated to around 36 kya,  in Italy:

The main issue, chronologically speaking, has been to determine when the figures were painted on the cave vault, since the layers in which they were recovered only provide a terminus ante quem for their age, in other words, an upper limit for their age.

So, at first glance, there is no evidence for the age of these paintings beyond that of the layers in which they were recovered. However, in this study, Broglio et al. (2009) make the case that all the paintings date to the earliest Aurignacian at the site, that is to level A2. Historically, the dating of the earliest Aurignacian at Fumane has been hotly debated, but the authors present new dates for previously dated charcoal samples that have undergone, for the new dates, a new, more thorough pretreatment (i.e., ABOx SC).

This has provided two statistically equivalent age determinations of 35,640 +/- 220 and 35,180 +/- 220 BP for level A2. Interestingly, and fittingly in light of my recent post on the presentation of calibrated and uncalibrated radiocarbon dates, they conclude that, by reference to the calibration curve based on the Cariaco Basin data and the GISP2 Greenland ice core, that “the chronological data show that Protoaurignacian Unit A2 dates to between 43,250 to 40,500 BPGISP2, with an age of 41,000BPGISP2 being statistically more likely” (Broglio et al. 2009:760; my translation, emphasis added).

Back for a final word from John Hoffecker via Michael Balter:

And John Hoffecker, an archaeologist at the University of Colorado, Boulder, says that the data sets behind the new curve will allow a more-precise correlation between radiocarbon dates and prehistoric climate reconstructions based on Greenland ice cores and other proxy indicators of ancient weather.

Even before the adoption of the new curve, Hoffecker says, those data sets were suggesting that modern humans had moved into Europe about 45,000 calibrated years ago, much earlier than previously thought–and early enough for them to have had substantial contact with Neandertals over thousands of years.  Although the new curve is a major landmark, it is “definitely not the last word” in radiocarbon calibration, Reimer says. Her team is already planning an update for 2011, “as we learn more about the Earth’s carbon reservoirs and how they changed over time.”

If we have AMH in Europe by 45 kya, and Neanderthals living in the same place till around 20-25  kya, we’re looking at a co-existence of the two peoples that lasted between 20,000 and 25,000 years. Recent research on pigmented shells in Spain dating to c.50 kya has now blown away the idea that Neanderthals didn’t have the cognitive capacity for symbolic representation, and indicates to me at least that we’re in for a few more surprises – for example I find it hard to believe that no Neanderthal ever daubed paint with intent on a cave wall, or passed on the occasional fashion tip to the arriviste moderns.

So although I might not go so far to say that ‘they were us and we were them’, but I’m of the firm opinion that to deny the likelihood that Neanderthals and AMH exchanged more than sultry looks across nocturnal camp-fires, seems increasingly mistaken, especially given around 20,000 years with time on their hands.

Increased accuracy from radiocarbon dating should help pinpoint more clearly the exact chronology of what is known so far, while further discoveries will doubtless cause more controversy still, but the overall trend of news points us toward a greatly more complex situation in the MP and UP than evidenced by ‘The Human Spark – Becoming Us’, viewable in full over at PBS;  made in 2008, it clearly illustrates how out of touch many researchers were back then, and to some extent, probably still are.

image: Fumane Cave