• The Radiocarbon age scale vs the 'real' (calibrated) years age scale.

    Most information on the past 30,000 years or so is from sites or specimens that have been dated using radiocarbon (14C). However, the radiocarbon age scale that would be calculated from first principles (based on the decay rate of the 14C isotope, assuming that 14C was at the same level of abundance as it is at present) is not always reliable, because there have been fluctuations in the rate of production in 14C at the top of the atmosphere. The problems are particularly great at about 10,000 14C y.a., when a large influx of 14C-depleted carbon from the oceans, combined with a decrease in the rate of 14C production at the top of the atmosphere, gives an 'age plateau' such that the same 14C age covers a wide span of real time, about 1,000 years. Other dating methods (e.g. U/Th) can be used to attempt to check the 'true' age of specimens or sediment layers dated by 14C, although these all have substantial error margins of their own. The most convincing way to check the 14C age scale is through biological or sedimentological features which build up annual layers over long periods of time (e.g. tree rings, and annual layers of sediment building up on lake beds); counting back the annual layers will reveal the true number of years before the present, and comparing the 14C age of each tree ring or sediment layer will give an age scale for how 14C age can be converted into 'real' age. However, even this method is not completely reliable; 'false' double rings can sometimes appear, and occasionally a year may not appear in the record. Because of these problems, individual ring or layer-counting studies often suggest 'real' ages differing from one another by several percent, though they all suggest that the 'real' age is older than the 14C age before about 3,000 years ago. The most recent working consensus (adopted by papers in leading Quaternary journals e.g. Dahl & Nesje 1996) 14C-to-real age conversion scale is given below, but because it is possible that opinions on the appropriate age conversion will change as more data come in, the time slices of the maps are presently described according to a 14C age scale. The reader can use this preliminary age scale as a guide to the likely true age of each of the time slices and vegetation distributions given on this page and on the QEN Pages. Useful sources on the current understanding of the radiocarbon timescale include Kilian et al. (1995), Goslar et al. (1995) and Stuiver et al. (1993). For times before 12,000 14 y.a., the table is mainly based upon the multi-source curve published by Kitagawa & van der Plicht (1998).

    It is necessary to bear in mind that quite apart from all the problems of calibration, a significant proportion of radiocarbon dates are not reliable for any purposes, because they have been contaminated with older or younger carbon that changes the apparent age of the sample. Many radiocarbon-dating specialists still refer to their field as 'more an art than as science'! Published radiocarbon dates from sites and layers of fossils and sediments are quite often rescinded, when the materials are found to have been naturally contaminated. Most often the contamination is from older (less 14C-rich) calcium carbonate, coal or charcoal washed in from other layers, making a sample or layer seem older than it actually is. Although radiocarbon dating is a very useful tool for the Quaternary palaeoecologist, it must always be interpreted with caution.

    14C years ago=>Calibrated ('real') years ago

    1,000 => 1,000

    2,000 => 2,000

    2,500 - 2,800 => 2,600 (sudden shift in atmospheric 14C content)

    3,000 => 3,200

    4,000 => 4,500

    5,000 => 5,900

    6,000 => 6,950

    7,000 => 7,900

    8,000 => 8,900

    9,000 => 10,000

    10,000 => 11,200 - 12,200 ('radiocarbon plateau')

    11,000 => 12,900

    12,000 => 14,000

    13,000 => 14,500

    15,000 => 17,000

    16,000 => 19,500

    17,000 => 21,000

    18,000 => 22,500

    20,000 => 24,500

    25,000 => 28,000

    30,000 => 35,000

    40,000 => 45,000