Table

Summary of Poles Used in Geomagnetism and Paleomagnetism

Point on the Earth's surface where the magnetic inclination is observed to be +90° (-90°). The poles are not exactly opposite one another and for epoch 1995 lie at 78.9°N, 254.9°E and 64.7°S, 138.7°E.

Point where the axis of the calculated best fitting geocentric dipole cuts the surface of the Earth in the northern (southern) hemisphere. The poles lie antipodal to one another and for epoch 1995 are calculated to lie at 79.3°N, 288.6°E and 79.3°S, 108.6°E.

The position of the equivalent geomagnetic pole calculated from a spot reading of the paleomagnetic field direction. It represents only an instant in time, just as the present geomagnetic poles are only an instantaneous observation. The pole of the paleomagnetic field averaged over periods sufficiently long so as to give an estimate of the geographic pole. Averages over times of lO3 years or longer may be required. The pole may be calculated from the average paleomagnetic field direction or from the average of the corresponding VGPs.

controlled by the rotational axis and has an equator to pole distribution. It is warmer at the equator than at the poles. The paleolatitude spectra of various paleoclimatic indicators should all be appropriately latitude dependent to be consistent with the complete GAD hypothesis. Results from this type of investigation are discussed more fully in §6.3.4.

1.2.4 Archeomagnetism

Pottery and (more usefully) bricks from pottery kilns and ancient fireplaces, whose last dates of firing can be estimated from 14C contents of ashes, have a TRM dating from their last cooling. Samples used in such studies, despite often having awkward shapes, can be measured by the usual techniques of paleomagnetism. Pioneer work in this field was undertaken by Folgerhaiter (1899) and Thellier (1937). The techniques commonly in use are those developed by Thellier and have been reviewed by Thellier (1966). Archeomagnetic investigations from different parts of the world are summarized in Creer et al. (1983) and are discussed in Merrill et al. (1996).

Cox and Doell (1960) observed that the average of VGPs calculated from observatory data around the world is close to the present geomagnetic pole. Unfortunately, archeomagnetic data are not evenly spaced around the world but are concentrated in the European region. Barbetti (1977) suggested that, to estimate the position of Recent geomagnetic poles, the effects of nondipole field

North (south) magnetic pole

Geomagnetic north (south) pole

Virtual geomagnetic pole (VGP)

Paleomagnetic pole variations could be averaged out if VGPs were averaged over 100-yr intervals for a limited number of regions of the Earth's surface.

The suggestion of Barbetti (1977) has been used by Champion (1980), Merrill and McElhinny (1983), and most recently by Ohno and Hamano (1992), who calculated the position of the North Geomagnetic Pole for successive times at 100-yr intervals for the past 10,000 yr. The results of the analysis of Ohno and Hamano (1992) are illustrated in Fig. 1.13 for each successive 2000-yr interval as well as for the entire 10,000 yr interval. Interestingly, the successive positions of the poles for 1600 to 1900 A.D. lie close to and have the same trend as the positions of the geomagnetic pole calculated from historical observations (Barraclough, 1974; Fraser-Smith, 1987; see §1.1.4 and Fig. 1.9b). Therefore, it seems reasonable to assume that 100-yr VGP means are indeed representative of positions of the North Geomagnetic Pole. Figure 1.13 shows that the mean VGP for each 2000-yr interval does not always average to the geographic pole, whereas the mean over 10,000 yr appears to do so. Thus, it appears that an interval of at least 10,000 yr is required for the dipole axis to average to the axis of rotation. Some caution is needed, however, because it is not at all clear that the motion of the dipole axis over the past 10,000 yr, as depicted in Fig. 1.13a, a) 10000-0 B P.

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