Archaeobotanical Remains: New Dating Methods
INTRODUCTION
Since the mid-nineteenth century, the disciplines of archaeology and botany have collaborated to form an important tool in the study of crop plant origins. The importance of this collaboration has always been enhanced by some means of dating the plant remains that were recovered. In regions such as Italy and Greece or the Middle East, classical texts and historic reconstruction aided in dating. In the American Southwest, dating of ruins by means of tree ring counts served a similar function. Today, radiocarbon dating is the most important method of determining the age of organic materials.
RADIOCARBON DATING
In 1949 W. F. Libby and associates at the University of Chicago showed that organic remains of known age from ancient sites in Egypt, Syria, and the American Southwest could be accurately dated by assessing their content of radioactive carbon. Since that time radiocarbon dating has become a foundation technique in archaeology and an important tool in the study of ancient plant remains. Radiocarbon dating is based on the ability to measure the proportion of the radioactive isotope 14C (carbon-14) to the stable (nonradioactive) isotopes 12C (carbon-12) and 13C (carbon-13) in a sample of organic matter. The rate of decline of any radioactive isotope is stated by its ‘‘half-life.’’ The radioactive isotopes of other elements decay more rapidly or less rapidly than carbon-14, whose half-life is about 5730 years, but the half-life of each type of radioactive element whether rapid or slow is always the same. Because of this, it is possible to determine the age of ancient plant remains and other organic materials.
TRADITIONAL AND ACCELERATOR DATING
Traditional radiocarbon dating, as developed by Libby, measured the emission of beta particles from wood charcoal, or some other organic material and is now sometimes called ‘‘indirect dating.’’ Because the amount of carbon-14 in such material is very low to begin with, and continually grows smaller, a fairly large sample is required and the detectable elapsed time from the death of the organism to the present is limited to about 50,000 years. The degree of error in the measurement increases with age. Another method of carbon-14 dating makes use of instruments (a particle accelerator and a mass spectrometer) combined in a system developed by physical scientists in the late 1970s.
The system is called accelerator mass spectrometry (AMS), or sometimes ‘‘direct dating’’ and has the ability to detect and count very small amounts of radiocarbon in a specimen. Both methods are destructive, that is, the sample to be dated must be destroyed in the process of dating. Early in the development of radiocarbon dating, it was assumed that the proportion of carbon-14 to other forms of the element had remained constant up until the time of extensive atomic bomb testing. The year 1950 was accepted as the end of constant proportions of carbon-14 in the atmosphere. It soon became apparent, however, that there were differences, or an offset, between age in ‘‘radiocarbon years’’ and age in actual years due to the lack of constancy of carbon-14 levels in the atmosphere.
CALIBRATION
In order to determine the magnitude of the offset, growth ring samples of the remarkably long-lived bristlecone pine and the giant sequoia were radiocarbon dated after an actual date had been determined by counting the annual rings of the trees. In order to provide regional standards, the procedure was repeated with other trees and other methods of dating that were applied to diverse organisms including corals. These tests showed that in the past there were variations in the amount of carbon-14 in the atmosphere, probably due to changes in the earth’s magnetic field.
As noted previously, more recent changes in the proportion of atmospheric carbon-14 have resulted from the testing of atom bombs. Knowing the magnitude of these variations in atmospheric radiocarbon allowed the age in radiocarbon years to be corrected to give the actual or calibrated age of a specimen. The result of carbon-14 determinations of the age of archaeological plant remains is reported in uncorrected or uncalibrated age as radiocarbon years before the present time (b.p.), where the present time is set at 1950. The calibrated age is reported in calendar years; hence, one-half of a seed of Phaseolus acutifolius (tepary bean) from a cave in the Tehuaca´n Valley of Mexico dates by AMS to 2300 ± 50 b.p. in radiocarbon years and 400– 210 B.C. in dendrocalibrated calendar years.
ACCELERATOR MASS SPECTROMETRY DATES: ADVANTAGES
The great advantage of AMS, over traditional radiocarbon dating, is the ability of AMS to give accurate dates on very small samples, for example, on one-half or less of a single maize kernel. The method is also much more rapid than traditional radiocarbon dating. Prior to the development of AMS the amount of early crop plant material retrieved from a particular level in an archaeological site was seldom sufficient for radiocarbon dating and this was later found to lead to errors.
For example, when a piece of wood charcoal excavated in the late 1940s (prior to the development of AMS dating) from Bat Cave in New Mexico was radiocarbon dated to about 6000 b.p. by the traditional method, that date was assigned to other materials associated with it. Among these other materials to receive the date of 6000 b.p. were small maize ears (not sufficient material to be dated by the traditional or indirect C-14 method). These small ears, because of their age, size, and other characteristics, resembled hypothetical wild maize, which was presumed to be extinct.
Years later small samples of the Bat Cave maize were dated by AMS and found to be not older than 3120 b.p. This brought the Bat Cave maize more into line with the earliest dates for prehistoric maize in other Southwestern sites, but it also demonstrated that traditional radiocarbon dates based on associated material could lead to erroneous conclusions. The disturbance of Bat Cave remains by burrowing rats meant that different objects in the same stratum may vary widely in age. Furthermore, because ‘‘smaller’’ and ‘‘older’’ did not necessarily go together, the smaller ears were not necessarily more primitive.
EARLY AGRICULTURE IN MEXICO
The pursuit of archaeological evidence for the origin of maize brought famed archaeologist R. S. MacNeish to Mexico, where he led major excavation projects, especially in the Tehuaca´n Valley south of Puebla during the early 1960s. It was in Mexico that the wild growing nearest relatives of maize had been found by botanists. MacNeish and his colleagues were successful in recovering important collections of maize, beans, squashes, and other cultivated and wild plant remains. The strata of prehistoric cave deposits were dated by traditional radiocarbon methods and the results of the excavations were widely recognized as the most important to have been made for the understanding of the origin of major American crop plants.
Excavation levels dated within a cultural period extending from 7000 to 5350 b.p. contained small maize ears, which in the opinion then prevalent among maize experts matched the hypothetical wild form. Published in 1967 the radiocarbon dates from the time of these earliest maize ears to the most recent, just prior to the Spanish conquest, were widely cited as the most authoritative archaeological timeline for the domestication of maize and the development of agriculture and human society in the high cultures of Mexico. When AMS dating became available, tiny samples of the ancient maize ears themselves were dated and were found to be from 4700 b.p. or about 1500 years younger than the radiocarbon dates previously obtained by the indirect method.
These more recent dates are especially significant for understanding the beginnings of Mesoamerican agriculture because they appear to reduce the perplexingly long time period between the first presence of maize and the period in which it became the primary component of the human diet. Accelerator dates on beans recovered from the same sites in the Tehuaca´n Valley were, like the maize dates, much younger than the dates on associated remains had indicated. In fact, the oldest accelerator dates for beans at Tehuaca´n were about 2300 b.p., which shows that, contrary to the historic unity of maize and beans in the Indian diet, there was a substantial period in which maize became a mainstay of the diet without the presence of beans.
PREMISE CULTIVATION IN EASTERN NORTH AMERICA
The value and reliability of AMS date on small samples of seeds from sites where disturbance caused critically important specimens to be mixed with earlier or later charcoal or other organic remains is widely recognized. One of the most significant applications of AMS dating is found in east-central North America where seeds and fruit parts of a suite of four indigenous plants showing characteristics of domesticates were AMS dated to the period of 2000–1000 B.C. The structural characteristics developed during this time period are regarded by most archaeologists specializing in eastern North America as evidence for an agricultural system predating the adoption of maize as the primary crop in this region.[8] The AMS dates of prehistoric maize from this region showed it to be later than previously thought, which fell in nicely with the proposal of a premise eastern North American cultivation system based on indigenous plant species.
AMS APPLIED TO AFRICAN AND ASIAN CROP PLANTS
The complex and still controversial evolution of domesticated maize contrasts with the evolution of African and Asian cereal grains where the wild ancestors of the domesticates are known and there has been a less structural transformation in the evolution of the domesticated crop. Nevertheless, AMS dating now plays an important role in assessing the process of domestication in Africa (especially sorghum) and Asia (especially rice). Carbonized rice grains embedded in ceramic potsherds from east central China[9] were AMS dated to 6400–5800 b.p. In this instance, the antiquity of the pottery, a non-organic ceramic material, as well as the rice, was determined.
CAUTIONS
Radiocarbon dates obtained by AMS and by traditional radiocarbon dating do not always disagree. Nor, when they disagree, are the AMS dates always more recent.
CONCLUSION
The application of structural and taxonomic botanical methods to the analysis of archaeological plant remains furthered the understanding of crop plant origins from the mid-nineteenth century until the present time. When the botanical analysis was combined with radiocarbon dating methods, the study of crop plant origins took a great leap forward.