Radiocarbon Dating And Quaternary Extinctions

The technique of radiocarbon dating, developed by Willard F. Libby in the 1940s and for which he was awarded a Nobel Prize in 1960, brought a revolutionary change to our understanding of many biological and physical events of near time. No other geochemical dating method is as powerful in aiding our understanding of dynamic changes over the past 50,000 years. Radiocarbon dating allows scientists to make the most accurate estimates possible for the timing of late-Quaternary extinctions. These estimates, in turn, are crucial in testing various models or ideas about the possible causes of near-time extinctions.

All living (organic) matter contains carbon, as does the Earth's atmosphere. Radioactive carbon, or radiocarbon (14C), is a low-energy radioactive isotope, or variant, of carbon that is continuously being formed in the upper atmosphere by the action of cosmic radiation on nitrogen-14 (14N). As do all radioactive molecules, the molecules of 14C subsequently decay at a characteristic rate. The ratio of 14C to nonradioactive carbon (12C ) in the atmosphere is very small.

As dynamic elements of nature's carbon cycle, living organisms maintain a small level of 14C in their tissues, in the same ratio to 12C as exists in the atmosphere. When an organism dies, the amount of radioactive carbon it contains is no longer replenished by exchange with the environment, and the amount of this radioactive carbon thus begins to diminish, decaying at a rate that reduces the number of 14C atoms by half over the span of 5,730 years. This time span is called the "half-life" of 14C. Radiocarbon dating measures the ratio of 14C to 12C in fossilized organic material and compares it to the ratio of 14C to 12C in the atmosphere. The difference is a function of the time since death. The older a specimen is, the less radioactivity it will retain. Measurements beyond 40,000 years (or seven half-lives) are possible but very difficult to obtain because of the heightened risk of older samples becoming contaminated (R. Taylor 1987).

Radiocarbon measurements are refined by comparing them to calendar years. For example, scientists can compare the I4C measurements to a date determined by dendrochronology, or tree-ring dating. Or they can compare the measurements to the layers of annually laminated sediments, or to other organic material of known age, such as wood from tombs of the pharaohs.

Each age estimate is accompanied by an error range (plus or minus a certain number of years), which is partly a function of sample size and partly a function of the time it takes to make the measurement. In the past 25 years, the use of particle accelerators has made it possible to analyze (at somewhat higher cost than with previous methods) much smaller samples in much less time. This is an advantage because much less of the specimen needs to be sacrificed to obtain a date, and the risk of contamination is lower.

Radiocarbon dating assumes that the ratio of I4C to I2C in the atmosphere has remained constant over time. There have been fluctuations, however. One such fluctuation resulted from the combustion of fossil fuels such as coal and petroleum during the Industrial Revolution. This released large amounts of I2C (nonradioactive carbon) into the atmosphere, reducing the I4C :I2C ratio. A much stronger shift in the opposite direction began with the testing of nuclear weapons; by 1963, nuclear tests had increased the atmospheric levels of I4C by over 90 percent. In both cases, the balance of nonradioactive carbon to I4C in the atmosphere was thrown off, impairing the accuracy of radiocarbon dating for specimens originating from the past two centuries. Trees that began growing in the mid-nineteenth century, or that predated but lived through the atomic age, are, according to radiocarbon dating, "too old" by 2 to 3 percent. With the elimination of atmospheric tests the production of I4C is returning to normal background levels, those produced only by cosmic rays.

Early in the refinement of the technique of calibrating radiocarbon dates with tree-ring dates, investigators noticed another problem: minor but persistent departures in their graphs from the expected straight line. These "squiggles" or "wiggles" of a few decades, sometimes termed the de Vries effect, are attributable to changes in the strength of the Earth's geomagnetic field, which provides a shield against cosmic radiation.

Also early in the development of radiocarbon dating, geologists and ecolo-gists recognized that carbonate samples taken from oceans or estuaries, and some lakes, would yield measurements that were too old. These water bodies are depleted of I4C because the carbon they receive comes from streams that

The Geologic Time Frame Calendar years before present (x 1000)

The Geologic Time Frame Calendar years before present (x 1000)

16 14 12 10 8 14C years before present (x 1000)

Figure 10. Relation between radiocarbon and calendar, or calibrated, years, from about 22,000 calendar years ago to the present. Note how the discrepancy increases over time. Reprinted from Klein 1999. Courtesy the University of Chicago Press.

16 14 12 10 8 14C years before present (x 1000)

Figure 10. Relation between radiocarbon and calendar, or calibrated, years, from about 22,000 calendar years ago to the present. Note how the discrepancy increases over time. Reprinted from Klein 1999. Courtesy the University of Chicago Press.

in turn receive their carbon from "old" rocks, which have relatively little 14C. When the inorganic carbon from limestone or other carbonate rocks becomes incorporated metabolically into organic material like aquatic plants or animals in sediments, such organic material is much "too old" when dated by its radiocarbon content. Even tissues from living aquatic animals in such 14C-depleted waters have been measured at thousands of years old.

In North America, mammoths and many other large mammals are thought to have gone extinct roughly two 14C half-lives (11,460 years) ago. Since the amount of 14C present in a specimen decreases by half during each half-life, this means that three-quarters of the 14C originally found in the organic residues of the last living mammoths had disappeared between the time these mammoths died and the time when their remains were collected and submitted for dating. However, the dating of megafaunal extinctions around the world remains controversial.

Through the past several decades of active use of the radiocarbon dating method, there has been an understandable tension between those eager for results on the samples they have submitted and those pretreating the samples and evaluating them for the possibility of contamination. As a general rule, the best-preserved bones (and occasionally tissues) for dating come from frozen ground or from dry caves, especially those in arid regions. However, fossil bones from salt-impregnated sediments, or sediments impregnated with petroleum residues, may retain relatively large amounts of well-preserved connective tissue (collagen), which can be contaminated by carbon depleted of 14C, thus throwing off radiocarbon measurement. Defensible dating requires removal of the petroleum residues from the specimens.

In continental North America it took at least two decades of measurements to discredit the belief that extinction of many large late-Quaternary mammals might have happened as late as 8,000 years ago or even more recently. Dates of this vintage continue to emerge from South America. Until recently, occasional Australian samples of extinct megafauna yielded measurements of less than 30,000 years ago, but these measurements have not been defended by replication at more than one laboratory. Of course, if many of the anomalously recent dates from continental North or South America or Australia prove to be defensible—that is, if they can be replicated by independent laboratories following established pretreatment protocol—a major readjustment in the theory of late-Quaternary overkill would be inevitable.

Another controversy centers on extinctions of relatively rare genera of large animals in North America. The few dates that have been posited from specimens of these animals are significantly older than 13,000 years. Those who believe that extinctions closely track human arrival discount these few older dates as insufficient for us to be confident that the genus in question became extinct significantly before the time of Clovis colonization, 13,000 years ago. Those who deny an anthropogenic role in the extinction process accept these dates at face value, suggesting that important megafaunal extinctions predated Clovis colonization.

The amount of I4C in the atmosphere at any one time is a function of what is held in the Earth's main carbon reservoir, the oceans. As explained earlier, the oceans are I4C-depleted (or I2C-rich) in relation to the atmosphere. During some time periods, there is a "degassing" of carbon from the oceans (the reasons are not well understood, but one cause may be a change in temperature). When this happens, the atmosphere receives a higher percentage of I2C (stable carbon) than is normal, diluting the amount of I4C. Organisms living during such a time period thus also contain less I4C than is normal, and that increases their apparent age when measured by radiocarbon dating. During the critical part of the time scale for extinctions in continental North and South America, from 10,000 to 13,000 I4C years ago, just such dynamic changes were under way in the oceans. Thus there is less precision than one would wish in the radiocarbon dating method across the three millennia when Clovis hunters arrived and large mammals became extinct.

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