compared with the present value of 7.9 x 10 Am for the best fitting dipole and 7.7 x 1022 Am2 for the axial dipole (g,° term).

6.3.4 Paleoclimates and Paleolatitudes

At the present time the mean annual equatorial temperature is about +25°C, and the polar value is about -25°C. Although the range in temperature from the equator to the pole may have varied in the past, the simplest model used by paleoclimatologists relates to the fact that the net solar flux reaching the surface of the Earth has a maximum at the equator and a minimum at the poles. Temperature therefore can be expected to follow the same pattern. The density distribution of many climatically sensitive sediments (climatic indicators) at the present time shows a maximum at the equator and either a polar minimum or a high-latitude zone from which the indicator is absent (such as reefs, evaporites and carbonates). A less common distribution, seen today in the distribution of glacial phenomena and some deciduous tress for example, has a maximum in polar and intermediate latitudes. Paleoclimatologists have demonstrated over the past few decades that the distribution of paleoclimatic indicators can be related to the present-day climatic zones that are roughly parallel with latitude (e.g. Parrish et al., 1982; Witzke, 1990). Therefore, if the expected latitude distribution of various paleoclimatic indicators can be generalized, it becomes possible to use statistical methods to estimate pole positions over geological time (Scotese and Barrett, 1990).

Irving (1956) first suggested that comparisons between paleomagnetic results and geological evidence of past climates could provide a test for the GAD hypothesis over geological time. The essential point is that both paleomagnetic and paleoclimatic data provide evidence for past latitudes and the factors controlling climate are quite independent of the Earth's magnetic field. The determination of the paleomagnetic pole for any region on the GAD assumption enables paleolatitude lines to be drawn across the region (§1.2.3). These are then compared with the occurrences of various paleoclimatic indicators. These comparisons can be made in four distinct ways.

(i) The time variation of paleolatitude for any single place may be compared with paleoclimatic evidence from that place (Blackett, 1961; Irving, 1956).

(ii) The space variations of paleolatitude over a given region for a given time may be represented as maps with the paleolatitudes and the occurrences of paleoclimatic indicators compared (Runcorn, 1961; Opdyke, 1962; Irving and Briden, 1962; Drewry et al., 1974).

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