Tag Archives: Radiocarbon dating

Calibrating 14C dating

Radiocarbon dating is the most popular tool for assessing the ages of archaeological remains and producing climatic time series, as in lake- and sea-floor cores, provided that organic material can be recovered. Its precision has steadily improved, especially with the development of accelerator mass spectrometry, although it is still limited to the last 50 thousand years or so because of the short half-life of 14C (about 5,730 years,). The problem with dating based on radioactive 14C is its accuracy; i.e. does it always give a true date. This stems from the way in which 14C is produced – by cosmic rays interacting with nitrogen in the atmosphere. Cosmic irradiation varies with time and, consequently, so does the proportion of 14C in the atmosphere. It is the isotope’s proportion in atmospheric CO2 gas at any one time in the past, which is converted by photosynthesis to dateable organic materials, that determines the proportion remaining in a sample after decay through the time since the organism died and became fossilised. Various approaches have been used to allow for variations in 14C production, such as calibration to the time preserved in ancient timber by tree rings which can be independently radiocarbon dated. But that depends on timber from many different species of tree from different climatic zones, and that is affected by fractionation between the various isotopes of carbon in CO2, which varies between species of plant. But there is a better means of calibration.

The carbonate speleothem that forms stalactites and stalagmites by steady precipitation from rainwater, sometimes to produce visible layering, not only locks in 14C dissolved from the atmosphere by rainwater but also environmental radioactive isotopes of uranium and thorium. So, layers in speleothem may be dated by both methods for the period of time over which a stalagmite, for instance, has grown. This seems an ideal means of calibration, although there are snags; one being that the proportion of carbon in carbonates is dominated by that from ancient limestone that has been dissolved by slightly acid rainwater, which dilutes the amount of 14C in samples with so called ‘dead carbon’. Stalagmites in the Hulu Cave near Nanjing in China have particularly low dead-carbon fractions and have been used for the best calibrations so far, going back the current limit for radiocarbon dating of 54 ka (Cheng, H. and 14 others 2018. Atmospheric 14C/12C during the last glacial period from Hulku Cave. Science, v. 362, p. 1293-1297; DOI: 10.1126/science.aau0747). Precision steadily falls off with age because of the progressive reduction to very low amounts of 14C in the samples. Nevertheless, this study resolves fine detail not only of cosmic ray variation, but also of pulses of carbon dioxide release from the oceans which would also affect the availability of 14C for incorporation in organic materials because deep ocean water contains ‘old’ CO2.

Improved dating sheds light on Neanderthals’ demise

As Earth Pages reported in December 2011 a refined method of radiocarbon dating that removes contamination by younger carbon has pushed back the oldest accessible 14C dates. Indeed, materials previously dated using less sophisticated methods are found to be significantly older. This has led archaeologists to rethink several hypotheses , none more so than those concerned with the relationship in Europe between anatomically modern humans (AMH) and Neanderthals, especially the extinction of the latter.

The team of geochronologists at Oxford University who pioneered accelerator mass spectrometry (AMS) of carbon isotopes, together with the many European archaeologists whose research has benefitted from it, have now published results from 40 sites across Europe that have yielded either Neanderthal remains or the tools they are thought to have fashioned (Higham, T. and 47 others. The timing and spatiotemporal patterning of Neanderthal disappearance. Nature, v. 512, p. 306-309) . One such site is Gorham’s Cave in the Rock of Gibraltar where earlier dating suggested that Neanderthals clung on in southern Iberia until about 25 ka. Another hypothesis concerns the so called Châtelperronian tool industry which previous dating at the upper age limit of earlier radiocarbon methodology could not resolve whether or not it preceded AMH colonisation of Europe; i.e. it could either have been a Neanderthal invention or copied from the new entrants. Most important is establishing when AMH first did set foot in previously Neanderthal’s exclusive territory and for how long the two kinds of human cohabited Europe before the elder group met its end.

Deutsch: Rekonstruierter Neandertaler im Neand...

Reconstruction of Neanderthal life from the Neandertahl Museum(credit: Wikipedia)

The new data do not quash the idea of Neanderthals eking out survival almost until the last glacial maximum in the southernmost Iberian Peninsula, since material from Gorham’s Cave could not be dated. However, occupation levels at another site in southern Spain in which Neanderthal fossils occur and that had been dated at 33 ka turned out to be much older (46 ka). So it is now less likely that Neanderthals survived here any longer than they did elsewhere.

Neanderthal remains are generally associated with a tool kit known as the Mousterian that is not as sophisticated as that carried by AMH at the same time. Of the Mousterian sites that yielded AMS ages, the oldest (the Hyaena Cave in Devon, Britain) dates to almost 50 ka. The youngest has a 95% probability of being about 41 ka old. Of course, Neanderthals may have survived until later, but there is no age data to support that conjecture. The earliest known AMH remains in Europe are those associated with the so-called Uluzzian tool industry of the Italian peninsula. In southern Italy Mousterian tools are replaced by Uluzzian between about 44.8 and 44.0 ka, while Mousterian culture was sustained in northern Italy until between 41.7 to 40.5 ka.

Châtelperronian stone tools

Châtelperronian stone tools (credit: Wikipedia)

Mousterian tool from France

Mousterian blade tool from France (credit: Wikipedia)

Châtelperronian tools associated with Neanderthal remains occur in south-western France and the Pyrenees. The new AMS dating shows that the culture arose at about the same time (~45 ka) as the Uluzzian tool industry began in Italy and ended in those areas where it was used at about the same time (~41 ka) as did the more widespread Mousterian culture. So the question of whether Neanderthals copied stone shaping techniques from the earliest Uluzzian-making AMH more than 500 km to the east, or invented the methods themselves remains an open question. But does it matter as regards the cognitive abilities of Neanderthals? Copying methodology is part and parcel of the success and survival of succeeding AMH, but o too is the capacity to invent useful novelties from scratch. So, yes it does matter, for Neanderthals had sustained the Mousterian culture for tens to hundreds of thousand years with little change.

The upshot of these better data on timing is that AMH and Neanderthals co-existed in Europe for between 2.6 to 5.4 ka; as long as the time back from now to the Neolithic and early Bronze Age. Even allowing for low population density to make contacts only occasional, this is surely too long for systematic slaughter of Neanderthals by AMH. Yet it gives plenty of time for two-way transmission of cultural and symbolic activities, and even for genetic exchanges: assimilation as well as out-competition.

Incidentally, Scientific American’s September 2014 issue is partly devoted to broader issues of human evolution (Wong, K. (editor) The Human Saga. Scientific American, v. 311(No 3), p. 20-75) with a focus on new developments. These cover: a revised time line; the emerging complexity of hominin evolution  by veteran palaeoanthropologist Bernard Wood.; the influence of climate change; by Peter de Menocal; cultural evolution in the broad hominin context by Ian Tattersall; a discussion of hominin mating arrangements by Blake Edgar; two contributions on cooperation versus competition among hominins by Frans de Wall and GGry Stix; two articles on recent biological and future cultural  evolution by John Hawks and Sherry Turkle (interview).