Galactic control of periodic events of Earth?
As Napier (1998) has pointed out, claims that the terrestrial record shows a periodicity of approximately 30 Ma have intermittently been made for at least the last 70 years, dating as far back as Arthur Holmes (1927). Such periodicity, it has been claimed, can be seen in climate variations, sea-level changes, mass extinctions, geomagnetic reversals and global volcanic episodes. It was first proposed by Napier and Clube (1979) that the cyclical changes cited above, together with ice-ages and plate tectonic processes, might ultimately be controlled by the galactic environment.
These authors assumed that the Oort cloud of comets was an intermediary, which was acted upon by our Galaxy, and was periodically disturbed during our Solar System's journey around the fringes of our Galaxy. These disturbances, they suggested, dislodge comets from the Oort cloud. Many of these are lost to outer space, but some are thrown inward towards the planetary system, and so gave rise to episodes of bombardment on Earth, which may endure for a few million years.
The Galactic hypothesis, which has been further developed by Clube and Napier (1982, 1984, 1986, 1996) and also by Rampino with various colleagues (1984, 1986, 1992, 1994, 1997), is based on the relatively recent findings that (a) the Oort comet cloud is unstable in the galactic environment and (b) the expected rates of collision between comets and Earth are now estimated to be sufficiently high that such periods of bombardment would result in 'geological trauma' on Earth.
For decades, it was generally considered that the major reservoir of potential Earth-impactors was the asteroid belt. However, beside the probability that asteroids from this belt are likely to give rise to impact craters of less than 20 km in diameter, there is no obvious mechanism which could be invoked that would give rise to a periodicity which has acted over a very long time. For such an extended impact scenario, 'the Earth becomes essentially a uniformitarian stage on which the drama of random impact is occasionally enacted' (Napier, 1998).
If, however, disturbances of the Oort cloud are periodically involved, then a more continuous process can be envisaged, which not only involves the direct effects of occasional cometary impacts, but also the effects of 'stratospheric dusting'. This latter effect can be ascribed to the fact that the mass distribution of comets is 'top heavy'. That is, most of the flux into the inner planetary system comes in the form of rare, giant comets —the disintegration of which leads to multiple impacts. In addition, Napier (1998) argues that when a large comet in an Earth-crossing orbit, with a diameter about, or in excess of, 100 km, breaks up, the finer particles created by this disintegration result in a prolonged 'stratospheric dusting' so that the 'optical depth' of the stratosphere may be increased for millennia (see also Bailey et al., 1994 and Clube et al., 1997).
Although it has been argued that such events may well be random (Alvarez et al., 1980); Napier and Clube (1979) suggested that these events are cyclical and that galactic periodicities must be expected to leave a terrestrial record with a periodicity of about 30 Ma. Other estimates of periodicity which relate to terrestrial impact craters are given in Table 8.1.
This periodicity of about 30 Ma is similar to that of successive vertical oscillations of the Sun as it passes through the plane of our Galaxy. However, as Napier pointed out, in the late 1970s, there was no known viable mechanism which connected our Galaxy to Earth. Nevertheless, it soon became clear that the vertical galactic tide (which is proportional to the density of the ambient material) was the dominant, though slowly fluctuating, external
Table 8.1 (After Napier, 1998.)
Seyfert and Sirkin (1979) Alverez and Muller (1984) Rampino and Stothers (1984) Shoemaker and Wolfe (1986) Yabushita (1994)_
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