Introduction And Overview

Accurate dating is of fundamental importance to paleoclimatic studies. Without reliable estimates on the age of events in the past it is impossible to investigate if they occurred synchronously or if certain events led or lagged others; neither is it possible to assess accurately the rate at which past environmental changes occurred. Strenuous efforts must therefore be made to date all proxy materials, to avoid sample contamination, and to ensure that the stratigraphie context of the sample is clearly understood. It is equally important that the assumptions and limitations of the dating procedure used are understood so that a realistic interpretation of the date obtained can be made. It is often just as important to know the margins of error associated with a date as to know the date itself. In this chapter, we discuss the main dating methods widely used for late Quaternary studies today. Further details can be found in Geyh and Schleicher (1990).

Dating methods fall into four basic categories (Fig. 3.1): (a) radioisotopic methods, which are based on the rate of atomic disintegration in a sample or its surrounding environment; (b) paleomagnetic (correlation) methods,4 which rely on past

4 One could argue that paleomagnetic changes do not constitute a method of dating but rather a method of stratigraphie correlation. Nevertheless, the development of a reliable timescale for paleomagnetic changes (Section 4.1.4) has meant that paleomagnetic changes are used, de facto, as dated reference horizons.

DATING METHODS

Direct ' Equilibrium measurement measurements of radioisotope (U-series)

RADIOISOTOPIC

DATING METHODS

BIOLOGICAL

Integrated I iwth effects I tes

track) I

Direct ' Equilibrium measurement measurements of radioisotope (U-series)

RADIOISOTOPIC

PALEOMAGNETIC

Integrated I iwth effects I tes

track) I

CHEMICAL

BIOLOGICAL

Growth layers (dendrochronology)

PALEOMAGNETIC

Organic (amino acid racemization)

Inorganic

Inorganic

Weathering 'Fingerprinting'

rates (tephrochronology) (obsidian hydration)

Incidence of Secular reversals variations

Weathering 'Fingerprinting'

rates (tephrochronology) (obsidian hydration)

FIGURE 3.1 Principal dating methods used in paleoclimatic research.

reversals of the Earth's magnetic field and their effects on a sample; (c) organic and inorganic chemical methods, which are based on time-dependent chemical changes in the sample, or chemical characteristics of a sample; and (d) biological methods, which are based on the growth of an organism to date the substrate on which it is found.

Not all dating methods provide a reliable numerical age, but may give an indication of the relative age of different samples. In these cases, it may be possible to calibrate the "relative age" technique by numerical (e.g., radioisotopic) methods, as discussed for example in Section 4.2.1.3. Thus, there is a spectrum of approaches to dating: numerical age methods; calibrated age methods; relative age methods; and methods involving stratigraphic correlation (Colman et al., 1987). In this and the following chapter, all of these approaches are discussed, beginning with numerical age methods.

Atoms are made up of neutrons, protons, and electrons. For any one element, the number of protons (the atomic number) is invariant, but the number of neutrons may vary, resulting in different isotopes of the same element. Carbon, for example, exists in the form of three isotopes; it always has six protons, but may have six, seven, or eight neutrons, giving atomic mass numbers (the total number of protons and neutrons) of 12,13, and 14, designated 12C, 13C, and 14C, respectively. Generally each element has one or more stable isotopes that accounts for the bulk of its occurrence on Earth. For example, in the case of carbon, 12C and 13C are the stable isotopes; 12C is by far the more abundant form. It is estimated that the carbon exchange reservoir (atmosphere, biosphere, and the oceans) contains 42 X 1012 tons of 12C, 47 X 1010 tons of 13C, and only 62 tons of 14C. Unstable atoms undergo

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