Feldman: When did interest in a possible variation in 14C arise?
Damon: One of the basic assumptions of radiocarbon dating was that the concentration of radiocarbon in atmospheric carbon dioxide had been constant in the past (see review by Damon, 1987 and Damon et al., 1978 for discussion and references). The first evidences of variation in the concentration were from anthropogenic causes. Suess (1955) first demonstrated a decrease in atmospheric radiocarbon concentration resulting from the combustion of fossil fuels and by 1957 it became apparent that the atmospheric 14C concentration was increasing as the result of atomic bomb tests (Rafter and Fergusson, 1957). However, Hessel deVries was the first to demonstrate that radiocarbon had fluctuated due to natural causes (deVries, 1958, 1959). In analyzing the phenomena, he used Craig's five box exchange model in formulating an electric analog representing the distribution and transport of radiocarbon in nature. In the process he identified two causes of the variation of atmospheric 14C variation. These were changes in the carbon cycle and changes in production rate resulting from solar activity (deVries 1959). It is interesting that monitoring of the neutron flux had just begun in 1956 but deVries was in personal communication with Simpson (Chicago) and Winckler and Ney (Minneapolis) who told him that they had "found that the total amount of neutrons produced in the higher atmosphere (integrated over the whole earth) had decreased by a factor of 2 in the last four years, therefore the natural production of 14C also decreased by this factor" (deVries, 1959, p. 187). This decrease may have given deVries an exaggerated impression of the typical variation of the global neutron flux because of the February 23, 1956, solar flare. In that year the annual production rate increased to 4.5 14C atoms/cm2esec and fell to 2.2 14C atoms/cm2esec during 1957 and 1958 and increased to 2.4 14C atoms/cm2esec during the year of the deVries publication, 1959 (Lingenfelter and Ramaty, 1970). During this seminal paper deVries had identified two of the three most important causes of atmospheric 14C fluctuation. The third cause was the decrease of the geomagnetic dipole intensity that resulted, during that period of time, in an increase of 0.14% per century increase in 14C concentration. His data did not cover a sufficient period of time to detect the geomagnetic effect.
Feldman: What roles did deVries and his student Minze Stuiver play in investigating 14C variation? Can you give an account of Stuiver's analog computer and its function in his research?
Damon: I have already discussed the electrical analog computer built by deVries (1959). Two pages from his article that describe the carbon cycle and electric analog follow. Hessel deVries, if it were not for his tragic death, would have continued to be ahead of all us, including Han Suess who never referenced the work of DeVries. DeVries' initial professorial appointment was in Nuclear Physics. Later on his interests shifted to Biophysics and he became a professor in Biophysics. Minze Stuiver's Ph.D. was in Biophysics with a masters in Nuclear Physics and Mathematics. DeVries was Minze Stuiver's Biophysics Ph.D. advisor. The electric analog model was used by Minze to demonstrate the relationship between radiocarbon and the ca. 11-year Schwabe cycle (Stuiver, 1961, 1965).
Feldman: What role did you play in the early investigation of 14C variation, say to 1970? What were your chief motivations in taking up this problem?
Damon: I arrived at the University of Arizona in time for the fall semester of 1957. The Geology Department had been criticized by the Physics and Chemistry Departments as a good pick and hammer department but behind the times. Urey, a chemist, had initiated the use of stable isotopes in geology, making use of Neir's advances in mass spectrometry. Neir, a physicist, had advanced geochronology; Blackett, a physicist, had initiated the field of geomagnetism; and Libby, a chemist, had developed 14C dating. I was supposed to bring the department into the post-World War II modern age of science with the commission to build an up-to-date 14C laboratory and to build a K Ar laboratory. At Lamont Geological Observatory, I had experience in geochronology but none in 14C dating. The first year was spent in designing the new laboratories prior to the availability of a new geology building. A chemist had built an already obsolescent 14C laboratory based on Libby's screen wall counter and turned it over to the anthropologists, who unfortunately had no training in physics or chemistry. Because the counter used a thin layer of elemental carbon from a slurry, it was easily contaminated by atmospheric C. Aerosols from the atomic bomb tests at the Nevada Proving Grounds greatly increased the danger of contamination of the carbon. I spent some time lowering the background and improving the electronics and sample preparation. We published one greatly improved date list but as soon as the new building was available, we tore it down and scavenged some of the parts such as the iron and lead to build the castle for proportional counters. The construction was financed by the Research Corporation and monitored by Dr. Harold Ramsey of the Research Corporation who fortunately seemed always to find me blowing glass or working on the electronics when he visited. Because of my previous lack of experience, I decided to confirm Libby's half-life of 5568 years. We did this by obtaining samples of acacia wood from the base of fortresses built during the reign of Sesostris III. We chose acacia wood because it is short-lived and from the base of fortresses because the thick base remains intact even after the superstructure is repaired. We chose the reign of Sesostris III (12th Dynasty) because during the seventh year of his reign there was a heliacal rising of the star Sirius. Parker, from Chicago, used this heliacal rising to fix in time the Egyptian Middle Kingdom and the periods preceding it. The dates that we obtained were always too young using the Libby half-life. We then obtained bristle cone pine tree rings dendrochronologically dated to 3791 - 3815 B.P. and obtained a date of 3260 ± 60 years B.P. At first we attributed this solely to Libby's half-life being incorrect. It later turned out to be 5730 ± 30 years (Mann et al., 1961; Watt et al., 1961; Olsson et al., 1962). The change in half-life yields only a 95-year increase in age. The remaining 95-year discrepancy turned out to be primarily the result of a changing geomagnetic dipole moment. This was the third major cause of atmospheric 14C fluctuation. After failing to confirm the Libby half-life, our literature search uncovered the work of Hessel deVries. My reaction was, of course, who could expect the atmospheric 14C/12C to remain constant? We then obtained tree rings from ponderosa pine and obtained AD 1596 to 1609 (D = 8±5ä), AD 1688 to 1692 (D = 22±5ä) and AD 1795 - 1804 (D = 2±4ä). Hence, we could conclude that there was an increase in D14C during the Maunder Minimum at 95% confidence. Our early work was financed by the Research Corporation only to build the laboratory. We had no other support. We were concerned about obtaining financing other than on a service basis. This was rectified by a Research Corporation unrestricted research grant (1960) that came as an unsolicited surprise. This amounted to $22,000 with the condition that no overhead should be applied. This was a considerable amount of money in 1960 dollars and enabled us to do sufficient work to be able to obtain NSF support. There followed a series of papers in Radiocarbon in which we confirmed the high level of 14C during what we would later find out from Jack Eddy was the Maunder Minimum, named it the deVries Effect and added to our work on 14C and the Egyptian calendar (Damon and Long, 1962; Damon, Long and Sigalove, 1963; Damon, Haynes and Long, 1965). This last was followed a year later by a JGR article putting it all together. Incidentally, after the tragic death of Hessel deVries, the deVries Effect became widely known as the Suess wiggles, although we stuck with deVries Effect. Han Suess never referenced deVries or Willis, Tauber and Munnich (1960) who extended the deVries Effect back through the past 1300 years and suggested that there was a ca. 200-year cycle. By 1968, although I refuted Han Suess's infatuation with Opik's speculation that low solar activity resulted in the ultimate ice age (1965, Climate change in cosmic perspective, Icarus, v. 4, 280-289-307), we both agreed on the following (Suess, 1967, Damon, 1968): " 1) According to theoretical prediction by Lingenfelter (1963), there should be an inverse relationship between C14 production o(Q,Ð) and the number of sunspots. 2) Such a relationship has been experimentally demonstrated by Stuiver (1961, 1965) for the 18th and 19th centuries. The data of Suess (1965) suggest that this relationship is consistently valid for short-term fluctuations during the last millennium. 3) There is also a relationship between climate and the C14 content of the atmosphere during the 16th through the 18th centuries, i.e., during the 'Little Ice Age.' The C14 content of the atmosphere appears to be high during the period of lower prevailing temperatures (Fig. 1). 4) This suggests that 'the sun is responsible for both kinds of changes and that the sunspots are symptoms of the state of the sun which affects both the cosmic-ray flux and the terrestrial climate.' 5) During the first few millennia B.C., the atmospheric C14 content is decreasing towards the present (Suess, 1965; Damon et al., 1966) as shown in Fig. 2. 6) Therefore, we may be observing the decay of a higher C14 inventory which existed during the last glacial episode when the solar activity was lower and o(Q,Ð) was consequently higher. In short, the suggested relationship may be summarized as follows. A large number of sunspots is symptomatic of high solar activity, including an increase in the solar constant which causes terrestrial warming and vice versa. The higher solar activity is also coincident with decreased o(Q,Ð) due to solar modulation of the galactic cosmic-ray flux." (Damon, 1968). By the time of the 12th Nobel Symposium on "Radiocarbon Variations and Absolute Chronology" held in August 1969 in Uppsala, Sweden, a number of us were in agreement that changes in the geomagnetic dipole moment were the dominant cause of 14C fluctuation, whereas the deVries Effect were 'wiggles modulating the geomagnetic trend (Bucha; Lingenfelter and Ramaty; Damon; Suess, 1969; John Wiley and Sons, 657 p., 1970, same title, ed. Ingrid Olsson). However, in my opinion, the most interesting paper in that symposium was the paper entitled "Astrophysical and geophysical variations in 14C production" by Lingenfelter and Ramaty. They not only included the effect of changes in the geomagnetic dipole moment but also added production of 14C by solar flares and supernova. We have published on evidence for both processes and are continuing this work and always reference Lingenfelter and Ramaty (1970).
Feldman: What were the chief sources of support for your work? How was your work related to the activities of the Laboratory of Tree-Ring Research at the University of Arizona?
Damon: As I have already mentioned, the Research Corporation provided the seed money and the NSF continuing support. Dendrochronology has been absolutely essential to calibration of the radiocarbon time scale and radiocarbon geophysics-astrophysics back to 11,000. The beauty of dendrochronology is with effort they can establish a tree-ring chronology accurate to one year. See the History of Calibration of Radiocarbon Dated by Dendrochronology (Damon, 1987) and particularly the section entitled "Dendrochronology: The importance of tree-ring dated samples in radiocarbon calibration," p. 68-70.
Feldman: Research into solar variability and its influence on climate is an interdisciplinary enterprise. What contribution did disciplinary crossover and cross-fertilization among disciplines make to the emergence of the field? Of what significance was it that both A.E. Douglass and Jack Eddy were astronomers? What contributions derived from interactions between physics and geochemistry/geological radiochemistry?
Damon: See "Development of the radiocarbon method of dating" (Damon, 1987, p. 62-64) and also table 4. All of the development up to 1951 was by chemists or physicists with one exception, Kulp's group at Lamont Geological Observatory, who were geochemists although Kulp himself had a Ph.D. in physical chemistry (see Milestone 11, 1951, and 16, 1959, Broecker and Olsson). Suess was a chemist; Willis, Tauber and Munnich were physicists (Milestone 16). Stuiver's Ph.D. was in biophysics. My degrees are physics (B.Sc.), geophysics (M.Sc.), geology-geochemistry (Ph.D.). Mann, Watt, Rafter, Klein, Olsson, Pearson, deJong et al., Mook and Kramer, all were physicists. See highlighted p. 457-459 for interaction between disciplines. Also, see the enclosed "Radiocarbon as an example of the unity of science" (Damon, Nobel Symposium, 1970, p. 641-644). A. E. Douglas' inspiration came when he was a circuit judge. He went from town to town by horse and buggy. One noon he sat on a large Douglas fir stump to eat his lunch and became fascinated with the alternating narrow and wide tree rings. He contemplated the cause and arrived at climate change. Then the idea came to him that climate change might be caused by changes in solar activity. He then designed a mechanical optical computer for two purposes. The first was to look for periodicities and he came up with the sequence 11, 22, 44 and 88-year periods. The second was the then popular idea that the gravitational tug of the planets, in particular, Jupiter, was modulating solar activity. He corresponded with Maunder. However, dendrochronologic dating became so effective that his solar interests were secondary. When I came here in 1953, the Navajo Indian Tribe was contributing $40,000 a year to the Laboratory for Tree-Ring Research because they were entitled to all the land that they occupied at the time of their treaty with the United States. If by dating of logs in a hogan they could prove that the hogan was built before the treaty, they could sue for possession. Unfortunately, the younger researchers were embarrassed by the interest of Dr. Douglas in solar activity. This was a backlash from the solar cycle 'freaks' of the 40s and 50s. See attached Reminiscence for a volume dedicated to Minze Stuiver's retirement this year.
Jack Eddy's contribution was to provide the historical perspective of the Maunder Minimum that had all but been forgotten. He also made interesting but non-rigorous paleoclimatic correlations. This had been in the wind since at least the 60s (e.g., see Suess, 1967; Damon, 1968). Of course, it is not surprising that an astronomer should recall the work of Maunder and other astronomers.