Physics applied to archaeology I. Dating

Abstract

An account is given of the three main chronological applications of physics in archaeology: radiocarbon dating, thermoluminescence dating and archaeomagnetism. In addition the following techniques are outlined: potassium-argon dating, uranium series dating (including ionium dating), fission track dating and dating by chemical change (including hydration of obsidian and weathering layers on glass). Some mention of dendrochronology and varve chronology is included in the section on radiocarbon.Radiocarbon dating, applicable to wood and some other organic remains, is predominant in this field and forms the basis of most prehistoric chronologies. Its techniques of measurement are now well established and the main research emphasis is concerned with the fluctuations in the radiocarbon content of the atmosphere that have been inferred from small systematic errors shown up in the dating of known-age samples. Possible causes for these fluctuations are past variations in climate, in sunspot activity and in geomagnetic field intensity. There is evidence from archaeomagnetic studies that the latter has been significantly different from its present day value, possibly varying sinusoidally with a maximum about 2000 years ago and a minimum about 6000 years ago. Archaeomagnetism, which is based on the magnetization `fossilized' in clay when it is fired, also provides data on the past secular variation of the geomagnetic direction and to a limited extent this can be used for dating; the geophysical interpretation of the data is also of interest. Thermoluminescence is a fairly recently developed method and the main research emphasis is still concerned with various complications of technique. It is important because it is directly applicable to pottery and pottery styles are the `grammar' of most archaeological chronologies. Potassium-argon dating is a well developed geological technique and is notable in the present context for its dating of volcanic material associated with early hominid remains. Uranium series dating has the possibility of being applicable to bone and shell of the paleolithic period, this being too early to be reached by radiocarbon. Fission track dating is primarily a geological technique but it holds promise for archaeology, particularly in early periods. Obsidian dating and glass layer counting are based on the hydration caused by weathering. The former needs to be calibrated against radiocarbon but in a limited number of cases the latter appears to be absolute although the difficulty of understanding the physicochemical mechanism involved casts some doubt on its validity.