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Frequency (cycles per million years)

Figure 8.8 Fourier spectrum of times series shown in Figure 8.7 (curve c). Curves R and W are estimates of spectral background. Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Rohde and Muller 2005), copyright © 2005.

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Age (million years)

Figure 8.9 Diversity of short-lived and long-lived genera. This plot, which is not detrended, shows the diversity of all genera that have both a first and last appearance resolved at stage or substage level and persisted (a) for 45 million years or less, and (b) more than 45 million years. Genera with only single occurrences were excluded. Vertical dashed lines indicate the times of maxima of the 62-million-year sine wave of Figure 8.7 (curve d). Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Rohde and Muller 2005), copyright © 2005.

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Age (million years)

Figure 8.9 Diversity of short-lived and long-lived genera. This plot, which is not detrended, shows the diversity of all genera that have both a first and last appearance resolved at stage or substage level and persisted (a) for 45 million years or less, and (b) more than 45 million years. Genera with only single occurrences were excluded. Vertical dashed lines indicate the times of maxima of the 62-million-year sine wave of Figure 8.7 (curve d). Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Rohde and Muller 2005), copyright © 2005.

mass extinctions (Loper et al. 1988). Moreover, episodes of major flood-basalt outpourings over the last 250 million years, when subjected to time-series analysis, displayed a possible periodicity of roughly 30 million years (Rampino and Stothers 1986). A more sophisticated analysis of 154 large igneous provinces revealed, among other things, several significant cycles at about 25-33 million years (Prokoph et al. 2004). Laboratory simulations of mantle plumes show relaxation oscillator modes, with plumes reaching the surface at regular intervals for six to nine cycles (Schaeffer and Manga 2001). This kind of behaviour in the Earth could drive periodic volcanism, but not necessarily at the desired period.

3 Another possibility is the up-and-down motion of the Solar System about the Galactic plane. The period of oscillation about the Galactic plane is roughly 67 million years, with estimates of the period varying between 52-74 million years (see Innanen et al. 1978; Bahcall and Bahcall 1985). Because of this bobbing motion, the Solar System passes through the Galactic plane (mid-plane crossings), where interplanetary matter tends to be denser, every 26-37 million years, and reaches its maximum distance (about 80-100 parsecs) from the Galactic plane every 33 million, too. The period decreases to half when the Solar System meets a higher-density Galactic arm.

4 Solar cycles could affect climate, but long-period oscillations in theory do not occur (Bahcall 1989).

5 Earth orbital oscillations could affect climate, but Rohde and Muller used an orbital integration package and nine point-mass planets (with no obliquity changes included) and found no significant cycles with periods of 62 million or 140 million years.

6 The Sun may have one or more companion stars, which might trigger periodic comet showers (p. 47). However, Rohde and Muller (2005) argue that a 62-million-year orbit is unstable to perturbations from passing stars, and that although the interaction of two or more short-period companions could generate a longer periodicity, their simulations suggest mutual perturbations would probably destroy any regularity.

7 Another suggestion is that an undiscovered tenth planet orbiting in the region beyond Pluto produces comet showers near the Earth on the right sort of timescale (p. 47). Until recently, no evidence for such a planet existed but a planet larger than Pluto was discovered on 8 January 2005 and named Sedna (after the Inuit goddess of the ocean). It possibly has a moon.

The discovery of a 140-million-year cycle was unforeseen. It has uncertain significance, although it matches the periods of cycles reported in climate (Veizer et al. 2000) and in cosmic rays (Shaviv 2002, 2003), a possible explanation being that crossings of galactic spiral arms at approximately 140-million-year intervals alters the cosmic ray flux, which alters climate (Shaviv and Veizer 2003) (see p. 69).

It is not yet certain that the 62-million- and 140-million-year cycles register variations in true diversity or only in observed diversity, but they require explanation and they imply that 'an unknown periodic process has been having a significant impact on Earth's environment throughout the Phanerozoic' (Rohde and Muller 2005, 210).

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