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Fig. 1.14. Global dipole moment versus time estimates obtained from 500-year period averages from 0-4000 yr B.P. and then 1000-year period averages . The number of measurements averaged is shown at each point together with 95% confidence bars. After McElhinny and Senanayake (1982).

The mean dipole moment for the past ten 1000-yr intervals is 8.75 x 1022 Am2 with an estimated standard deviation of 18.0%, which may be attributed to dipole intensity fluctuations. There is a maximum around 2500 yr B.P. and a minimum around 6500 yr B.P. Cox (1968) had previously thought that the data summary as illustrated in Fig. 1.14 was indicative of variations in the dipole moment with a simple periodicity of between 8000 and 9000 yr with maxima and minima respectively about 1.5 and 0.5 times the present dipole moment. However, the data available for times before 10000 yr B.P. show clearly that this is not the case and data for the interval 0-5 Ma are also inconsistent with the expectations of a periodic variation (Kono, 1972; McFadden and McElhinny, 1982; Merrill eta/., 1996).

1.2.5 Paleointensity over Geological Times

The problems of determining the paleointensity of the geomagnetic field are much more complex than those associated with paleodirectional measurements and become increasingly difficult the older the rocks studied. The presence of secondary components and the decay of the original magnetization all serve to complicate the problem. Kono and Tanaka (1995), Tanaka et al. (1995), and Perrin and Shcherbakov (1997) analyzed all the available measurements in terms of VDMs. In Fig. 1.15, the best estimate of the variation of the Earth's dipole moment over the whole of geological time (Kono and Tanaka, 1995) is summarized for the past 400 Myr averaged at 20-Myr intervals (Fig. 1.15a) and prior to that at 100-Myr intervals (Fig. 1.15b).

Prévôt et al. (1990) first suggested that there was an extended period during the Mesozoic when the Earth's dipole moment was low, at about one-third of its Cenozoic value. Further measurements for the Jurassic (e.g. Perrin et al., 1991; Kosterov et al., 1997) supported low values. During the Cenozoic the dipole moment was similar to its present value. For the period prior to 400 Ma, the number of paleointensity measurements is much fewer. Of particular interest is the oldest paleointensity measurement, which is for the 3500 Ma Komati Formation Lavas in South Africa (Hale, 1987). The average VDM of 2.1±0.4 x 1022 Am2 is about 27% of the present dipole moment. This result clearly demonstrates the existence of the Earth's magnetic field at 3.5 Ga. The range of variation of the Earth's dipole moment is about 2 - 12 x 1022 Am2 and is approximately the same for Phanerozoic and Precambrian times (Prévôt and Perrin, 1992). With the present data set, no very-long-term change in dipole moment is apparent. Kono and Tanaka (1995) point out that it is remarkable that the dipole intensity seems to have been within a factor of 3 of its present value for most of geological time.

Fig. 1.15. Variation of VDM with geological time, (a) The past 400 Myr averaged over 20-Myr intervals, (b) The past 3500 Myr averaged over 100-Myr intervals. Error bars give the standard error of the mean for each interval and the number of measurements in each interval is indicated. The horizontal dashed line in (b) indicates the present dipole moment (8 x 1022 Am2). After Kono and Tanaka (1995).

Fig. 1.15. Variation of VDM with geological time, (a) The past 400 Myr averaged over 20-Myr intervals, (b) The past 3500 Myr averaged over 100-Myr intervals. Error bars give the standard error of the mean for each interval and the number of measurements in each interval is indicated. The horizontal dashed line in (b) indicates the present dipole moment (8 x 1022 Am2). After Kono and Tanaka (1995).

1.2.6 Paleosecular Variation

Secular variation of the geomagnetic field in pre-archeological times has been investigated through paleomagnetic studies of Recent lake sediments. Long-period declination oscillations in cores taken from the postglacial organic sediments deposited at the bottom of Lake Windermere in England were first discovered by Mackereth (1971). Since that time, many such studies have been made throughout Europe, North America, Australia, Argentina, and New Zealand. Such studies are generally referred to as studies of paleosecular variation (PSV). Extensive investigations of lakes in England and Scotland have enabled a master curve of changes in declination and inclination in Great Britain over the past 10,000 years to be determined (Turner and Thompson, 1981, 1982) as illustrated in Fig. 1.16. Further details on such studies and their interpretation are summarized in Creer et al. (1983) and discussed by Merrill et al. (1996).

The GAD model takes no account of secular variation, although its effect must be averaged out before paleomagnetic measurements are said to conform with the model. The secular variation in paleomagnetic studies is expressed by the statistical scatter in paleomagnetic results after the effects of experimental errors have been removed. To estimate this scatter it is necessary to be sure that each measurement is a separate instantaneous record of the ancient geomagnetic field. Sediments cannot readily be used for this purpose because even small samples may have already averaged the field over the thickness of sediment covered by

Declination -40 -20 0 20 40 J_ ■ I ■ I ■ I ■

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