mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== ORAU [1]Oxford Radiocarbon Accelerator Unit > [2]Information on ORAU > [3]Research and applications > [4]^14C measurement services > [5]^14C results > [6]OxCal calibration software > [7]Information on ^14C University of Oxford _________________________________________________________________ 14C Info [8]Information on radiocarbon > [9]Sources of radiocarbon > [10]Radiocarbon dating > [11]AMS measurement > [12]Radiocarbon calibration Radiocarbon Calibration * [13]Why radiocarbon measurements are not true calendar ages * [14]How tree rings are used as a radiocarbon record * [15]How radiocarbon calibration works * [16]Typical calibrated ranges * [17]Some conventions * [18]Calibration programs * [19]Calibration curves * [20]Further reading _________________________________________________________________ Why radiocarbon measurements are not true calendar ages Radiocarbon measurements are always reported in terms of years `before present' (BP). This figure is directly based on the proportion of radiocarbon found in the sample. It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years. For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made. Schematic of radiocarbon dating To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP. For two important reasons, this does not mean that the sample comes from 3619 BC: * firstly the proportion of radiocarbon in the atmosphere has varied by a few percent over time * the true half life of radiocarbon is 5730 years not the original measured value of 5568 years In order to see what a radiocarbon determination means in terms of a true age we need to know how the atmospheric concentration has changed with time. _________________________________________________________________ How tree rings are used as a radiocarbon record Schematic of dendrochronological dating Many types of tree reliably lay down one tree ring every year. The wood in these rings once laid down remains unchanged during the life of the tree. This is very useful as a record of the radiocarbon concentration in the past. If we have a tree that is 500 years old we can measure the radiocarbon in the 500 rings and see what radiocarbon concentration corresponds to each calendar year. Using very old trees (such as the [21]Bristlecone Pines in the western U.S.A.), it is possible to make measurements back to a few thousand years ago. To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns. By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region. By using dead trees of different but overlapping ages, you can build up a library of tree rings of different calendar ages. This has now been done for [22]Bristlecone Pines in the U.S.A and waterlogged Oaks in Ireland and Germany to provide records extending back over the last 11,000 years. _________________________________________________________________ How radiocarbon calibration works Calibration of radiocarbon determinations is in principle very simple. If you have a radiocarbon measurement on a sample, you can try to find a tree ring with the same proportion of radiocarbon. Since the calendar age of the tree rings is known, this then tells you the age of your sample. In practice this is complicated by two factors: * the measurements on both the tree rings and the samples have a limited precision and so there will be a range of possible calendar years * given the way the atmospheric radiocarbon concentration has varied, there might be several possible ranges These effects are most clearly seen by looking at a specific example. Example calibration plot This plot shows how the radiocarbon measurement 3000+-30BP would be calibrated. The left-hand axis shows radiocarbon concentration expressed in years `before present' and the bottom axis shows calendar years (derived from the tree ring data). The pair of blue curves show the radiocarbon measurements on the tree rings (plus and minus one standard deviation) and the red curve on the left indicates the radiocarbon concentration in the sample. The black histogram shows possible ages for the sample (the higher the histogram the more likely that age is). The results of calibration are often given as an age range. In this case, we might say that we could be 95% sure that the sample comes from between 1390CalBC and 1130CalBC. See also ORAU's [23]Explanation of Radiocarbon Results and [24]Typical Calibrated Ranges. _________________________________________________________________ Some Conventions This is not intended to be an exhaustive summary of radiocarbon calibration conventions but a brief guide. * [25]Reporting radiocarbon dates * [26]The calibrated time scales * [27]Methods of calculating ranges _________________________________________________________________ Reporting radiocarbon dates Radiocarbon dates should always be reported either as `percent modern' or years `before present' (BP). The first indicates the proportion of radiocarbon atoms in the sample as compared to samples modern in 1950. The second is directly derived from this on the assumption that the half-life of radiocarbon is 5568 years and the amount of radiocarbon in the atmosphere has been constant. _________________________________________________________________ The calibrated time scales Once calibrated a radiocarbon date should be expressed in terms of calBC, calAD or calBP. The cal prefix indicates that the dates are the result of radiocarbon calibration using tree ring data. These values should correspond exactly to normal historical years BC and AD. The term calBP means the number of years before 1950 and can be directly compared to calendar years. _________________________________________________________________ Methods of calculating ranges There are two main methods used for calculating age ranges from the calibration curve: The first method to be employed was called the `intercept method' because it can be done by drawing intercepts on a graph. This method will tell you the years in which the radiocarbon concentration of tree rings is within two standard deviations of your measurement (e.g. between 2940BP and 3060BP for the measurement 3000+-30BP). A slightly different method is now more often used which is called the `probability method'. This requires a computer since the calculations are more complicated. It gives the time range, from which you can be 95% sure the sample came. _________________________________________________________________ Calibration programs There are a number of calibration programs available including the windows program [28]OxCal and CALIB, which runs on several platforms including an online version (servers at [29]Washington and [30]Belfast). _________________________________________________________________ Calibration curves The main calibration curves that have been used are given here. All of these data files will run with any version of OxCal: For most applications, with terrestrial material, the InCal98 calibration curve should be used. In the Southern Hemisphere an offset should be applied. The exact value is the subject of research (ee Stuiver et al 1998 for details). For example, for South Chile, New Zealand and Tasmania the offset seems to be 24+-3 ^14C yr. To apply this correction either subtract 24+-3 from the radiocarbon date prior to calibration, or apply a DeltaR correction of 24+-3 to the calibration curve. For marine (oceanic) samples the Marine98 curve should be used but you need to know about any regional offsets (see Stuiver, Reimer and Braziunas 1998 and Stuiver and Braziunas 1993). A very useful source of data on this is the [31]Marine Reservoir Correction Database at Queens University Belfast. Name Data Format Reference 1986 [32]Curve Groningen Stuiver and Kra 1986 1993 [33]Curve Groningen Stuiver and Kra 1993 IntCal98 [34]Curve Calib Stuiver et al 1998 Marine98 [35]Curve Calib Stuiver et al 1998 For the period after 1950, there is no inernationally agreed calibration curve. However, there is a great deal of data on the atmospheric radiocarbon concentration. A good source for this is the WWW site, [36]Atmospheric Carbon Dioxide and Carbon Isotope Records. See, for example, the plot of atmospheric data from Vermunt in Austria: [37]Curve Atmospheric data from Vermunt in Austria ([38]I Levin et al, Heidelberg) _________________________________________________________________ Further reading * Aitken M.J. 1990 Science-based dating in archaeology London, Longman * Stuiver M. and R.S. Kra eds. 1986 Calibration issue, Proceedings of the 12th International 14C conference Radiocarbon 28(2B) 805-1030 * Stuiver M., A. Long A., and R.S. Kra eds. 1993 Calibration issue Radiocarbon 35(1) * Stuiver M. and T.F. Braziunas 14C Ages of Marine Samples to 10,000 BC Radiocarbon 35(1) 137-189 * Stuiver and van der Plicht (eds) 1998 Calibration Issue Radiocarbon 40(3) * Stuiver M., P.J. Reimer, E. Bard, J.W. Beck, G.S. Burr, K.A. Hughen, B. Kromer, G. McCormac, J. van der Plicht and M. Spurk 1998 INTCAL98 Radiocarbon Age Calibration, 24000-0 cal BP Radiocarbon 40(3) 1041-1083 * Stuiver M., P.J. Reimer and T.F.Braziunas High-precision radiocarbon age calibration for terrestrial and marine samples 1998 Radiocarbon 40(3) 1127-1151 * [39]The online manual for OxCal * [40]Marine Reservoir Correction Database See also the [41]OxCal reference list. _________________________________________________________________ ISO-9001-2000 QAP 01/10 BSI Registered References 1. http://www.rlaha.ox.ac.uk/orau/index.html 2. http://www.rlaha.ox.ac.uk/orau/index.html 3. http://www.rlaha.ox.ac.uk/orau/research.html 4. http://www.rlaha.ox.ac.uk/orau/services.html 5. http://www.rlaha.ox.ac.uk/orau/results.html 6. http://www.rlaha.ox.ac.uk/orau/oxcal.html 7. http://www.rlaha.ox.ac.uk/orau/info14c.html 8. http://www.rlaha.ox.ac.uk/orau/info14c.html 9. http://www.rlaha.ox.ac.uk/orau/sources.html 10. http://www.rlaha.ox.ac.uk/orau/dating.html 11. http://www.rlaha.ox.ac.uk/orau/ams.html 12. http://www.rlaha.ox.ac.uk/orau/calibration.html 13. file://localhost/www/jnocook.net/saturn/new/calibration.html#radiocarbon 14. file://localhost/www/jnocook.net/saturn/new/calibration.html#tree_rings 15. file://localhost/www/jnocook.net/saturn/new/calibration.html#calibration 16. http://www.rlaha.ox.ac.uk/orau/01_06.htm 17. file://localhost/www/jnocook.net/saturn/new/calibration.html#conventions 18. file://localhost/www/jnocook.net/saturn/new/calibration.html#programs 19. file://localhost/www/jnocook.net/saturn/new/calibration.html#curves 20. file://localhost/www/jnocook.net/saturn/new/calibration.html#refs 21. http://www.sonic.net/bristlecone/intro.html 22. http://www.sonic.net/bristlecone/intro.html 23. http://www.rlaha.ox.ac.uk/orau/explanation.html 24. http://www.rlaha.ox.ac.uk/orau/typical_cal.html 25. http://www.rlaha.ox.ac.uk/orau/conventions_reporting 26. http://www.rlaha.ox.ac.uk/orau/conventions_timescale 27. http://www.rlaha.ox.ac.uk/orau/conventions_ranges 28. http://www.rlaha.ox.ac.uk/orau/oxcal.html 29. http://depts.washington.edu/qil/calib/ 30. http://radiocarbon.pa.qub.ac.uk/calib/ 31. http://www.qub.ac.uk/arcpal/marine/ 32. http://www.rlaha.ox.ac.uk/orau/cal10.dta 33. http://www.rlaha.ox.ac.uk/orau/cal20.dta 34. http://www.rlaha.ox.ac.uk/orau/intcal98.14c 35. http://www.rlaha.ox.ac.uk/orau/marine98.14c 36. http://cdiac.esd.ornl.gov/trends/co2/contents.htm 37. http://cdiac.esd.ornl.gov/trends/co2/graphics/cent-vegr.gif 38. http://cdiac.esd.ornl.gov/trends/co2/cent-verm.htm 39. http://www.rlaha.ox.ac.uk/oxcal/oxcal.htm 40. http://www.qub.ac.uk/arcpal/marine/ 41. http://www.rlaha.ox.ac.uk/oxcal/ref.htm