We can determine the time necessary for lakes to collect mud deposited by melting glaciers, for rivers to build their deltas, for waterfalls to cut their channels and to remove the bedrock, for lakes without outlets to accumulate their salts. We can ascertain how much time has passed since beaches were raised, by the state of their shells, and the age of volcanic rocks by the amount of erosion. By counting the annual bands of clay and silt we may find out the number of years spent in their deposition. By studying the rings in old tree trunks we can determine the time of climatic changes as reflected in their growth. The remains of extinct and extant animals - their appearance, position on the ladder of
evolution, and state of fossilization - enable us to establish their time of existence. By the content of radiocarbon in organic matter we may detect the time when an animal or plant died, and by the accumulation of fluorine in bones the length of time since burial. Finally, by studying artifacts and archaeologically determinable strata in the lands of antiquity, we may discover the time of deposit of associated animal or human remains; and by associated pollens of plants, a geochronological scale of climatic changes can be formulated even for areas where no archaeologically datable objects are found. There are a few other ways of calculating geological time: by measuring the amount of sediment on the bottom of the ocean; by computing the amount of salt in the oceans and comparing it with the annual influx of salts from land; and, finally, by the analysis of rocks for their lead content as a product of decay of radioactive elements. But these ways, especially the last two, cannot be profitably applied for measuring time in thousands or tens of thousands of years; they were devised for reckoning time in millions of years.
Of the methods used to find how much time has passed since the ice cover started to melt, the "varve" method, until recently, was thought to be fairly precise. This method was introduced by G. de Geer, who counted the annual bands of silt and clay ("varves") deposited, coarse in summer and fine in winter, under the ice in the coastal lakes and rivers of Sweden, once covered by the glacial sheet of the Ice Age. De Geer calculated that it had taken about 5000 years to melt the ice cover from Schonen, at the southern tip of Sweden, to the place in the north where there are still glaciers in the mountains. In no place are there five thousand overlying varves; but De Geer looked for similar series or patterns of thick and thin varves from one lake to another, about fifteen hundred outcrops altogether, always with the thought that a varve series found high in the deposit of some southern lake would repeat itself closer to the bottom of a lake to the north.
Additional figures used in De Geer's evaluation of the time that passed since the end of the Ice Age are of a more hypothetical nature. For the preceding period, the time allegedly needed for the ice to retreat all the way, from Leipzig to southern Sweden, where no varves are found, De Geer offered, as a surmise, a span of 4000 years. Then he surmised further that the end of the melting of the ice cover coincided with the beginning of Neolithic time, which he placed 5000 years ago, thus arriving at the final figure of 14,000 years, or 12,000 years before the present era. The area of Stockholm was freed from ice about 10,000 years ago. Other scientists freely interpreted De Geer's data as indicating that the ice cover in Europe started to melt 25,000 or even 40,000 years ago. 1 The method, when applied to
North America, also gave the figure the explorers looked for, namely 35,000 to 40,000 years; in this estimate great stretches of land without varves in them were freely evaluated as to the time in question. De Geer applied his method of identifying synchronical varves to countries as far apart as Sweden, Central Asia, and South America. His telechronology was objected to on the ground that a dry phase in Scandinavia may not necessarily have coincided with a dry phase in the Himalayas or in the Andes, and that therefore the telechronology was built on an erroneous assumption. 2 But the method as applied to
of varves from one dried-out lake to another is a delicate process and often subjective appraisals replace an objective method; especially arbitrary are the estimates for intervening stretches of land where no varves are found.
In 1947 an ingenious new method of investigating the age of organic remains was developed by W. F. Libby of the University of Chicago. The radiocarbon dating method is based on the fact that when cosmic rays hit the upper atmosphere they break the nitrogen atoms into hydrogen (H) and radiocarbon (Ch), or carbon with two extra electrons, therefore unstable, or radioactive.
The radiocarbon is mixed with the atmospheric carbon and as carbon dioxide it is absorbed by plants; it enters the animal body that feeds on plants and also the carnivore that feeds on other animals. Thus all animal and plant cells as long as they live contain approximately the same amount of radiocarbon; when death comes, no new radiocarbon is assimilated and the radiocarbon present in the remains undergoes the process of decay, as every radioactive substance does. After 5568 years only half of the radiocarbon remains; after another 5568-year period only half of the half, or a quarter of the original content in the organic body, remains. A sample undergoing analysis - a piece of wood or skin - is burned to ashes and its radiocarbon content is determined by a Geiger counter. This method claims accuracy for organic objects between 1000 and 20,000 years old; bones and shells are unsuitable materials because organic carbon is easily lost in the process of fossilization, often being replaced by carbon in ground water and by mineral salts.
The first important result of the radiocarbon dating method in glacial chronology was a radical reduction of the terminal date of the Ice Age. It was shown that ice, instead of retreating 30,000 years ago, was still advancing 10,000 or 11,000 years ago. 3 This conflicts strongly with the figures arrived at
by the varve method concerning the final phase of the Ice Age in North America. 4 Even this great
reduction of the date of the end of the Ice Age is not final. Radiocarbon analysis, according to Professor Frederick Johnson, chairman of the committee for selection of samples for analysis, 5 revealed
"puzzling exceptions." In numerous cases the shortening of the time schedule was so great that, as the only recourse, Libby assumed a "contamination" by radiocarbon. But in many other cases "the reason for the discrepancies cannot be explained." Altogether the method indicates that "geological
developments were speedier than formerly supposed." 6
H. E. Suess of the United States Geological Survey reported recently that wood found at the base of interbedded blue till, peat, and outwash of drift, and ascribed by its finder to the Late Wisconsin (last) glaciation, is, according to radiocarbon analysis, but 3300 years old (with a margin of error up to two hundred years both ways), or of the middle of the second millennium before the present era. Still more recently Suess and Rubin reported that "a glacial advance in the mountains of western United States was determined to have occurred about 3000 years ago." 7
Already there is an accumulation of similar results that do not fit into the accepted scheme, even if the Ice Age is brought as close to our time as 10,000 years. Professor Johnson says: "There is no way at the moment to prove whether the valid dates, the 'invalid ones,' or the 'present ideas' are in error." 8 He says
also: "Until the number of measurements can be increased to a point permitting some explanation of contradictions with other apparently trustworthy data, it is necessary to continue to form judgments concerning validity by a combination of all available information."
With this idea in mind, I offer in the following sections a review of the results of several other methods of time measurement, especially as regards the dating of the last glaciation.
Libby recognizes that the exactness of his method is dependent on two assumptions. The first is that for the last 20,000 or 30,000 years the amount of cosmic radiation reaching our atmosphere remained constant; the other is that the quantity of water in the oceans has not changed in the same period of time. Actually only a minor part of the radiocarbon created by cosmic rays is absorbed by plants and animals, the so-called biosphere; a still smaller part is present in the atmosphere; the largest share is absorbed by the ocean.
the past cosmic radiation could have reached the earth at a different intensity; and in a future book I intend to show that the waters of the oceans and their salts were increased substantially in a recent geological age.
Bearing in mind these limitations, I confidently expect that in the field of geology more and more "puzzling" results of radiocarbon tests will compel a full-scale revision of the dating of the glacial periods. 9
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1 Chamberlin, in The World and Man, ed. Moulton, p. 93; Daly: Our Mobile Earth, pp. 189-90, C. Schuchardt:
Vorgeschlchte von Deutschland (1943), p. 3.
2 E. Antevs: "Telecorreletion of Varve Curves," Geologisma Förhandlingar.1935, p. 47; A. Wagner: Klimaänderungen und
Klimaschwankungen (1940), p. 110.
3 F. Johnson in Libby: Radiocarbon Dating (1952) , p. 105.
4 Antevs: "Geochronology of the Deglacial and Neothermal Ages," Journal of Geology, LXI (1953), 195-230. Cf., however, G. de Geer in Geografiska Annaler, 1926, H. 4. He evaluated the time when the ice cover left the region of Toronto as about 9750 years ago.
5 The Committee on Carbon 14 of the American Anthropological Association and the Geological Society of America. 6 Johnson in Libby: Radiocarbon Dating, pp. 97, 99, 105.
7 Science, September 24, 1954, and April 8, 1955. 8 Johnson in Libby: Radiocarbon Dating, p. 106.
9 In the field of archaeology, I expect the radiocarbon tests to confirm that the time of the Eighteenth Dynasty in Egypt must be reduced by five to six hundred years, and the time of the Nineteenth and Twentieth Dynasties by a full seven hundred years, as I maintain in Ages in Chaos.
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