both temporal coverage and the types of samples that can be directly employed. The use of the K/Ar method and inferences from the paleomagnetic record combined with lithostratigraphic and biostratigraphic analysis have resulted in the establishment of basic chronological structures for the hominid record in the early Quaternary, especially for important sites in eastern Africa. For the late Quaternary—and particularly the most recent 30,00050,000 years—chronological frameworks are also reasonably complete in broad outline for many areas of the world, largely because this period represents the current practical time span that can be documented by the 14C method. Thus, although many problems remain, reasonably specific chronological frameworks exist for the hominid biological and cultural evolutionary record for both the Early Pleistocene and the terminal PleistoceneHolocene. In contrast, the dating frameworks for the period centered on the Middle Pleistocene, about 730,000125,000 years ago, are generally imprecise and ambiguous. Consequently, the vast majority of well-dated archaeological sites or paleoanthropological localities are older than one million years or younger than about 50,000 years. In 1979, Grant Raisbeck and Françoise Yiou proposed using a relatively long-lived (ca. 100,000 years) isotope of calcium, 41Ca, as the basis of a method to infer age for Quaternary bone and other calcium-containing samples (17). (Yoshio Yamaguchi had previously suggested the use of 41 Ca for dating in 1963 [30].) Like 14C, 41 Ca is produced by cosmic-ray neutron secondaries. As illustrated in Figure 6, however, the bulk of the radiocalcium is not made in the atmosphere as is the case with 14 C, but primarily in the upper meter of the soil profile by neutron capture on 40Ca. Following production, 41Ca would be mixed with the other naturally occurring calcium isotopes into the soil matrix through natural bioturbation processes and groundwater action. Calcium would be taken up into plant tissue in the form of Ca 2+ through ion absorption into the root system. Radiocalcium, like all of the other isotopes of calcium, would also be incorporated into bone mineral. A 41 Ca/ 40 Ca equilibrium ratio would be maintained in living organisms by exchange and metabolic processes. In contrast to 14C dating, in which the death of an animal or plant and the isolation of a sample from one of the carbon reservoirs constitutes the i=0 event (zero B. P. on the 14C time scale), the i=0 event in the projected 41Ca method would be the shielding of a sample from the effects of neutron irradiation, for example, by the placement of a sample at a depth in excess of
about three meters of soil or rock overburden either through burial or placement in a cave-rock shelter environment. Age inferences would be based on the measurement of the residual 41 Ca with respect to the stable Ca isotopes. The decay of 41Ca to 41K takes place by electron capture and an emission of a neutrino. Until the development of AMS counting technology, it was not practical to pursue research on natural level 41Ca measurements, primarily due to its expected extremely low terrestrial concentrations. The ratio of 41Ca to 40Ca was expected to lie in the range of 10~14 to 10~15. For comparison, the 14 C/ 12 C ratio in modern carbon is about 10~12. From a technological point of view, the 41 Ca method, in many respects, currently stands at the same point in its development as the 14C method did in the fall of 1945. The favored mode of production for 14C had been known for some time (thermal neutrons on 14N) and, based on this fact and several other considerations, a dating model had been formulated. In 1945, however, the half-life of 14C was still somewhat uncertain, and natural levels could only be measured after enrichment. Currently, the half-life figure of 41Ca is known only to about 10%-20%. The first direct measurement of the natural terrestrial concentration of 41Ca was carried out in April 1986 at the Argonne National Laboratory using the ATLAS linac accelerator system following the enrichment of 41Ca at the Oak Ridge National Laboratory using a Calutron isotope separator. The value obtained for 41 Ca/Ca was 2.0 ± 0.4 X 10~14 {31). Studies are currently underway to determine if the 41Ca method has the potential to provide a practical means of determining accurate age estimates for Pleistocene bone and other calcium-bearing samples.
From I Through Z, Just Some of the Compounds You Can Measure At Low Picogram Levels [ndole-3-Aeel«ldehyde Indoleaceiic Acid
Iso-Homovamllic Acid [so-VMA Kynucenlc Acid L-Dopa Leu-enkephalin Menadione 3 Mercaployeline (Penicillamine) Melanephnne Melaprolerenol Mel-enkephalin Methionine Melholrexate 3 Metho*y,4 Hydroxyphenylqlycol 3 Melho>y.4-HydiMiyphenylglycol Svliale 3.Math«y Indoleecehc Add 3.Meihuxyraeihyldopa 3-MethoxyphenylelhyUmim> 3-MT(3Melhoxylyramine) 3-Methyl-4-Hydroxyphenyl Glycol BMelhoxysalicylic Acid Meihyl Reserpate (Reserpme) Mel h y] para ben MHPG, MHPGSullale Morphine N-Melhylepinephrme N-Melhylnicolinamide N-Melhylserotomn N-Melhyltryplophan Ν Melhyllryplamine NMethyltyiamme Nalorphine Naloxone NE (Norepinephrine) Neuropephde γ Nicotine Nicotinic Acid Nuiad.-eiial.n* Norcodeme Norepinephrine [NE] Normelanephnne Noimc.phine Octopamine Op.a.es Ρ OH Phenylactlii A..-
Pteroylqlulamic Acid
Serotonin (5 HT) (S-Hyd.oxytryptamin.)
SET Τ (Triiodothyronine) T. (Thyroxine)
Ε. VanillylmandelicAcd Vamlmandel.c Acd «•••
.
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cid;
Conclusion The core issues of contemporary paleoanthropological and prehistoric archaeological studies revolve around the nature and causes of the evolution of hominid behavior over the last few million years as reflected primarily in the physical residue of that behavior. Evolution, of course, means change over time. It is precisely the ability to document and deal with human behavior within an extended temporal framework that allows prehistorians to make their unique contribution by focusing on the nature and causes of the evolution of social and cultural systems within our species and our biological antecedents. Contemporary prehistorians have come to have at their disposal an increasing array of physical dating methods developed by colleagues in the physical sciences. Because of the in-
Prove it for yourself. Call today for a demonstration of the Coulochem in your laboratory.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987 · 329 A
