Sherburne F. Cook

Archaeological Dating by Chemical Analysis of Bone

THE THEORY underlying a chemical dating system is quite simple. It rests upon the assumption that within human or animal bones, subsequent to burial, certain chemical changes occur in a progressive manner such that there is a continuous accumulation or depletion of substance. Chemical anaylses will then provide a series of figures with which the corresponding times and dates can be equated. Throughout the past several years the writers have been engaged in an attempt to determine whether the above theoretical approach can be justified in practice. They have subjected to various types of analysis several hundred bones from a series of nearly thirty archaeological sites within the state of California and have supplemented this series with shorter ones from the southwestern states, from New York, and from Kentucky. The results, from the standpoint of dating, are presented here. In order to construct an adequate dating system based upon chemical data two prerequisites must be satisfied. First there must be a continuous numerical scale, derived from the chemical analyses, according to which the data from a series of sites may be arranged. Such a scale, ranging from the earliest to the most recent times, will give us by itself the relative sequence of the sites. Secondly this scale must be tied, at least at two points, into the flow of chronological or clock time in order that we may get the absolute sequence of the sites and, by interpolation, thus permit the investigator to fix a date for each site. In so far as our investigations have taken us there is nothing in the chemical data per se which will permit the satisfaction of the second prerequisite. On the other hand different lines of approach make the task possible. With respect to the most recent period there is historical or protohistorical information which enables us to set the lower time limit, as it were, of the latest material. For the prehistoric period we are now in possession of sufficient key dates obtained by the radiocarbon method to pin down a minimum of sites with considerable accuracy. Dating by carbon-14 must therefore be the significant point of reference as far as absolute time is concerned. Since chemical determinations of bone from our group of California sites is the most extensive of which we have knowledge, it will serve as a test series. There are twenty-eight sites, for two of which (SJo-68 and Mnt-282) we have

New Evidence of Antiquity of Tepexpan and Other Human Remains from the Valley of Mexico

N JANUARY, 1957 Dr Luis Aveleyra Arroyo de Anda, Director of the Museo Nacional de Antropologia in Mexico, and Dr Arturo Romano, Director of the Department of Prehistory of the Instituto Nacional de Antropologia e Historia, Mexico, provided us with a series of bone samples of humans and extinct animals which ave been recovered under conditions strongly suggestive of contemporaneity and high antiquity. In each case some obscurity and uncertainty over contemporaneity, and therefore antiquity, exists, and it was the hope that ests for nitrogen content and fluoride l vel of the bones would indicate some solution to the problem of whether the human remains were younger (presumably b reason of intrusion i to older deposits) than the animals, or whether the human and animal remains were deposited at approximately the same time and are therefore coeval.l As one control we determined the nitrogen a d fluorine content of the femur from a human skeleton excavated by us at Tlatilco, a Preclassic site near Los Remedios on the outskirts ofMexico City.2 This burial was accompanied by a number of stone and ceramic offerings which indicate that he grave refers to the final stage of the Middle Preclassic with an estimated dating of 700 to 500 BC. Wood charcoal collected from soil surrounding the burial has been radiocarbon dated3 at 2525 ? 250 years old (568 BC). The much discussed human skeleton from Tepexpan was represented in our tests by a rib fragment. No animal remains are associated directly with the Tepexpan skeleton,4 but at a distance of about 2.6 km have been recovered two skeletons of mammoths, each of which was associated with flaked projectile points and other 1 A grant from the Wenner-Gren Foundation for Anthropological Research supported this investigation. The bone samples were secured incidental toa research trip financed by the National Geographic Society. A list of published papers dealing with the chronological significance of bone constituents is contained in an article by the present authors in this journal, Vol. 12, pp. 229-248, 1956. 2 On the Tlatilco site see Porter, 1953; Pifia Chan, 1958. 3 Sample M-660 (reported in Science, vol. 128, p. 1120, 1958). 4 On the discovery, geology, and physical nthropology of the Tepexpan skeleton see De Terra, 1947; De Terra, Romero and Stewart, 1949; De Terra, 1957, pp. 160-171; Aveleyra, 1950, chap. 3.