RADIOCARBON USERS CONFERENCE WELLINGTON 17th & 18th August 1971 RÄBIOCABBON USERS' CONíERENCE held at INSTITUTE OP NUCLEAR SCIENCES LOWER HUTT NEW ZEALAND 17TH to 18TH AUGUST 1971 Department of Scientific & Industrial Research New Zealand Government Printer 1 Aug 1971 Wellington 2a INDEX continued The Selection of Archaeological Samples for 196-205 Radiocarbon Dating L. Lockerbie Possible Use of Natural ffax Extracted from 206-209 Peat? as a Radiocarbon Sating Material T.L.Grant-Taylor The Reporting of Very Old Radiocarbon Dates 210-213 K. J. Gougfa FOREWORD This Radiocarbon Users conference was first suggested by Dr David Kear, Director of New Zealand Geological Survey and enthusiastically taken up by Dr Rafter, Director of the Institute of Nuclear Sciences. It was organised with a view to providing a single forum for many of the aspects that affect radiocarbon dates, hoping to avoid the pitfalls of tco great specialisation in any field. There are continuous developments as might be expectedt and quite a number of these developments have been made here at the New Zealand Laboratory, not only in the actual mechanical process of dating but also in the background philosophy, and understanding of lene natural processes affecting the carbon component of fossils. The New Zealand Laboratory was one of the first two or three to be established, and is now the oldest contin- uously operating radiocarbon laboratory in the world. The laboratory has been in existence now for about 20 years» our first date list being published in 1953» I wish to thank all who have helped me in the prep- aration for this conference, particularly those people who are presenting papers and especially Mrs E.M. Tiller of my office. ' T.L. Grknt-Taylor INDEX History and Concepts of Radiocarbon Dating 3-21 T. A. Rafter An Evaluation of Some Methods of Radiocarbon 22-28 Counting K. J. Gough Gas Counters and Counting 29-36 D.R. Currie Practionation of Carbon Isotopes by Higher Plante 37-46 J. H. Troughton Tree Rings and Radiocarbon Dates 4-7-51 H. S. Jansen The Presence of Humic Acias in Radiocarbon-Dated 52-58 Charcoal J.M.Bailey and K. S.Birrell Contamination of Radiocarbon Samples 59-64- T.L.Grant-Taylor Selection of Samples for Radiocarbon Dating for 65-70 Geological Purposes T. L. Grant-Taylor Radiocarbon Dating of Soil Organic Matter: 71-85 Its Scope and Limitations K. M. Goh The Use of Radiocarbon in Measuring the Turnover 86-98 of Soil Organic Blatter J.D.Stout Radiocarbon Dating in Botany 99-111 N. T. Moar Radiocarbon Chronology of Late Quaternary Rhyolite 112-116 Tephra Deposits C.G.Vucetich and ff.A. Pu"> lar Ages, inferred from 11+C Da tes,of some Tephra and 117-138 other deposits from Rotorua, Taupo, Bay of Plenty, Gisborne, and Hawke'a Bay Districts W.A.Pullar and Janice £. Heine 1UC Dates in the Quaternarj^Geology of t^e "Golden Coast".Wellington C. A. Fleming Identification of ffood from Tree Stumps and Drift 14.9-I56 flood associated witb Tephra Layers in Alluvium ani of Charcoal from Tephra Layers ff.A. Pullar and R.N.Patel The Use of Radiocarbon Dating in Marine Geology 137-164 J.V.Eade Radiocarbon Dating of Bone Organic and Inorganic 163-181 Matter H.Polach Degradative Methods as Aids in Soil Humic Acid 182-191 Characterisation K.R.Tate Low Temperature Ashing 192-195 P.C.Rankin 3. HISTORY ANO CONCEPTS OF RADIOCARBON DAi.% An address to the Radiocarbon Users Conference Wellington - August 17-18, 1971 by O T.A. Rafter SUMMARY The paper discusses some of the main controversial issues reported to the "Radiocarbon Variations and Absolute Chronology" symposium held at Uppsala from 11 to 15 August 1969. 14 The ccnst&ncy of the C production rate is one of the main. uncertainties in the reliability of the carbon-14 dating Method. How this is being estimated, and preliminary suggested corrections for carbon-14 against dendrochronological ages, are given. 4. In 1969 I was invited to attend the XII Nobel Symposium at Uppsala, where I presented four papers on the work of the Institute. The proceedings of the symposium were published recently in this rather elegant volume and I have reprints here of the four papers we presented. After the Uppsala Conference, I was invited to the ANZAAS Conference at Adelaide on my way home. I said that I would review the papers presented at Uppsala on Radiocarbon Variations and Absolute Chronology. I spent a wet Sunday in Zurich working on this review and a day before the lecture, preparing notes. Since then I have had little opportunity to go over these notes again, but I felt they would 09 worth revising for. this Radiocarbon Users Conference. Thirty-two papers were presented that could be loosely grouped into sessions devoted to: (1) Historical Chronology (£) l/arved Clay Chronology (3) Dendrochronology (4) Radiocarbon Reservoir Problems (5) Astro - and Geophysical Phenomena and their Effect on Radiocarbon Variations (6) Dating Methods and Related Analyses Dr W.F. Libby, in an open session at the conclusion of the Conference, gave an excellent review of the carbon-14 method. I think it would be appropriate here to briefly¿go over the principles of the method so that we can see more clearly the problems involved as far as our own interests are concerned. Carbon>14 is produced by cosmic rays coming from outer space that, bombarding the atmospheric gases, produce neutrons that, fortunately for us, are rapidly absorbed by nitrogen atoms, forming C-14 as illustrated by the equation 14 * n • 14W 14 • The excitad C atom, by a process we are still not sure about, oxidises to CO.. 14 • 14 * CO C • IL Carbon-14 is a radioactive species of carbon, disintegrating back to 14 N by the emission of a beta particle. Carbon-14 has a half-life of 55B0 years or a mean life of 8033 years and its decay follows what mathematicians call an exponential curve. 5600 11,200 Time This curve relates activity to time by an equation A. = A e that we need not worry about any further, t o 6. 14 The C0? molecules rapidly mix with the normal CCL molecules of the atmosphere and through the sun's energy and the photosynthetic process in the green leaves of plants is assimilated intu plants and so into animals and human beings such as you and me. We therefore have the relationship sunlight • 'CO--*- HO + plants —> complex organic compounds * The indicates that the organic compounds are tagged by a v/ery símil number of C-14 atoms present (actually only about one C-14 atom per 1012 D-12 atoms). If uie are to use the radioactive decay of carbon-14 as a method of dating, we must not only be confident that the decay rate is constant but also that the concentration of C-14 per unit weight initially was also constant. By this I moan that the number of C-14 atomg per gram of carbon synthesised by trees at the beginning of this Í¡ century is the same as that synthesised by trees 10, 20, 30 or 40,000 •t years agoc The concentration of C-14 is therefore controlled by the relationship ¿C-14/ = Cosmic ray production rate Pool of carbon 2n per cm /sec 8g carbon/cm the pool is made up approximately as follows: 2 living matter = 0.3 g per cm atmosphere oceans humus 7. Note that most of the carbon is in the oceans so that the circulation patterns of the norId*s oceans is of great importanos in what follows. WB have assumed that the C-14 production rate has remained constant over the last 40,000 years and that the C-14 produced was uniformly mixed in a few hundred years within the "pool" and finally, that after death on removal from its natural environment, the specimen be it human, plant, animal, shell or soil, is isolated from C-14 contamination with the anvironment, so that decay of C-14 is the only process that can vary 'che C-14 concentration. When the C-14 dating method was first developed, it was accurate to only _• 250 years and its reliability was checked against historically dated materials. Subsequently, the accuracy was improved to _• 50 years. Troubles were then experienced with historical specimens because in many cases the C-14 method dated only the time of removal of the carbon specimen from its natural environment, not the historicel event. These two events could be separated by hundreds of years, e.g., a coffin to date the death of an Egyptian king. This difficulty could at first be overcome by using short-lived materials, e.g., seeds, leaves, sandals, etc., bu*- the question that remained unanswered was the constancy of the C-14 production rate. An attempt to arrive at this was attempted by comparing the C-14 activity in trees, that had been accurately dated by the science of dendrochronology. In 1958 a Dutch scientist, ds vries showed for the first time that a secular effect existed, i.e., a set 14 of long-term changos in the contemporary concentration of CO. in 8, the atmosphere at a particular location, as well as differences that may exist between different locations. This work was extended by Suess of the University of California, San Diego, using the giant sequoia that had a tree-ring age over 2,000 years. The possibility of studying in more detail the C-14 variations in the atmosphere was greatly extended by Dr C.W.