The environmental effects of impacts
To clarify how some of the cyclical events may be related to impact events, let us consider the wider aspects that result from the impact of a major meteorite into the ocean. Roddy et al. (1988) reported the results of computer modelling of impact effects, in which it was assumed that the impacting body had a diameter of 10 km, travelled at a velocity of 20 km s-1 and made impact into a 5 km deep ocean, with an energy of 3. 0x1022 joules. Although this analysis was conducted before it was generally realised that cometary impacts gave rise to the larger impact features, the modelling is nevertheless instructive. (A degree of correlation between Roddy's modelling and that of a cometary body giving rise to a comparable energy of impact can be obtained if, to off-set the density difference of 3.3/1 between the two types of bodies, one assumes the initial cometary velocity to be about 45 km s-1.)
Their model showed that the immediate effect of the impact is to heat a large mass of air, to a peak temperature of 20,000° K, adjacent to GZ, which then moves out rapidly from the impact area. If a landmass were adjacent, such a 'fireball' would consume combustible materials. In the ocean, a devastating tsunami would be generated with an initial wave height determined by the depth of the ocean, so that the initial wave may be several kilometres high. This wave would spread from the impact area at an initial velocity of 0.5 km s-1. The energy of such a wave would be sufficient to scour the sea- or ocean-floor and rework sediments and develop tsunami deposits, particularly in the shallow waters of the continental shelf. Roddy claims that the volume of reworked material is so large that any ejecta material, including mineral or geochemical signatures from the meteorite or comet at the impact area, could be largely, or totally, obscured. Napier (personal comment) makes the point that whatever the type of impacting body, a very large proportion of the body's energy would certainly arrive at the impact site. Such an impact would result in an earthquake in excess of magnitude M=12 on the Richter scale (see Melosh, 1989 for the method of calculating M), and such energy would contribute to landslides and submarine slumping as well as initiating, or accelerating, volcanic activity and hydrothermal activity.
Roddy et al. estimate that somewhat in excess of 100 km3 is ejected from the crater area and they report that approximately 1014 tonnes of vapourised asteroid, or comet, and crust are thrown to high altitudes. It is thought that 80-90 per cent of these ejecta falls back in or near the crater. However, the remaining material is thrown to heights of about 100 km in the form of dust and ultra-small particles, which remain suspended in the upper atmosphere long enough to circle the globe. These particles block out sunlight and cool the Earth's surface (the oceans would be cooled by 3-4°C and the land surface by as much as 40°C). This would, of course, disrupt the food chain by reducing or preventing photosynthesis. It has been estimated that dust from such an event will not settle in less than 6 months. However, these estimates do not include the effects of soot particles, generated by the fireball, which are much smaller and so will settle more slowly. Soot absorbs light better than fine ejecta (Wolbach et al., 1985); nevertheless, it is thought, complete darkness is not likely to persist much beyond 6 months.
Yet another environmental consequence of a major impact is the shock-heating of the lower atmosphere with the ensuing formation of nitrous and nitric acid which, with their compounds, would inhibit photosynthesis by reducing solar radiation, and asphyxiate fauna through exposure to NO2 (Lewis et al., 1982; Prinn and Fegley, 1987).
It will be clear from these various effects that an impact capable of generating a crater with a diameter of 100-200 km can bring about catastrophic marine and terrestrial results which can give rise to extinction over considerable areas. One may readily imagine that the effects of an impact, several orders of magnitude more energetic than that considered by Roddy, upon Earth's environment must be even more awesome. Thus, the mass extinction of many forms of marine life and of many species of terrestrial flora and fauna must result from a really major impact (i.e. an impact which generates a crater with a diameter of several hundreds of kilometres). In addition, moderate size impacts in the shallow seas of the continental margins can easily give rise to such disruption as to be used to define sequence boundaries. (See McClaren and Goodfellow, 1990 for evidence of a catastrophic nature relating to the K/T and other boundaries back to the Precambrian.)
As regards more recent events, Stothers (1993) has even related seven stage boundaries to seven specific impact events, which range in diameter from 10 to 45 km. However, these relatively small impact structures are terrestrial, so it is difficult to envisage how they could give rise to stratigraphic stage boundaries. It is reasonable to assume that if these boundaries are indeed related to meteoritic impacts, they would most likely result from quite large, oceanic impact events.
The interrelationships between the various events given in Table 8.2 are statistically intriguing. It is also interesting to note that the then known impact events are not cited in this table. Nevertheless, with the aid of the Roddy scenario, it is evident that three of the classes of events (mass extinction, anoxic conditions and stage boundaries) can be explained in terms of major impacts.
Moreover, we have already argued that, as the result of a moderate-to-large impact, partial melting of the LVZ could result in such reduction of basal resistance to glide that the velocity of sea-floor spreading could be influenced. The violent motions of the ocean in the vicinity of an impact would be so intense that the event would cause at least a regional problem, as regards continuity of life forms. Changes of toxicity could also occur in the oceans. The terrestrial atmospheric conditions would be sufficiently toxic to render the life style of large land animals untenable. Despite the critical comments we have cited in an earlier paragraph, we do not doubt that the shock-wave of oceanic water must have a dramatic influence on the sediments and, thereby, would be likely to cause the initiation of a stage boundary. The presence of nodules and stressed minerals at the K/T boundary attest to this conclusion. In addition, Oberbeck et al. (1933) point out that the tsunamis associated with such major marine impacts could give rise to sediments which are remarkably akin to glacial, boulder-clay deposits.
Thus, of the important geological events cited in Table 8.2 only evaporite deposits and orogenic events have not yet been shown to be directly relatable to impact events. Orogenic events are reasonably well explained in terms of conventional plate tectonics, though a major impact could exert a significant trigger-mechanism in bringing about the initiation of an orogeny. As regards the remaining type of event, which relates to the development of salt and other evaporitic deposits, only three examples are cited in Table 8.2. Because such deposits require marine incursions and climatic conditions acting for long periods, we find it difficult to envisage an obvious direct relationship between impacts and the development of such major evaporite sequences.
Let us now consider sudden sea-level changes. We have already indicated how major oceanic plateau basalts (e.g. the Ontong-Java at 119.3 Ma) resulted in the emplacement of many tens of millions of cubic kilometres of basalt on the ocean floor. Let us assume that the area of the deep oceans has remained essentially unchanged in the last 250 Ma, so that we can take the current area of the ocean, not including continental margins, to be 3.1* 1014 m2. If we take the average depth of ocean to be 3770 m (after Turcotte and Schubert, 1982), then the 'constant' volume of the ocean is approximately 11.7*1017 m3 or 11.7* 108 km3. If we further assume that the volume of lava that erupted to form the Ontong-Java plateau basalt was 60*106 km3, then this represents approximately 5 per cent of the volume of the ocean and would give rise to an increase in sea-level of about 190 m.
However, we noted in Chapter 6 that in the initial stages of development of the Ontong-Java plateau basalt the erupted basalts were, in part, subaerial, so that one would expect this event to cause a sea-level rise perhaps significantly less than 190 m.
Harland et al. (1982) show an abrupt rise of sea-level at 116 Ma of approximately 100 m. These authors point out that the exercise they conducted in plotting eustatic changes in sea-level 'is not likely to be construed as presenting a consensus'. However, in light of the difficulty of assessing the precise age of onset and the magnitude of the change in sea-level, the estimated dates of change of sea-level and also the magnitude of the rises are probably in reasonable agreement with the impact date of this feature as inferred from its track anomaly. This abrupt eustatic increase in sea-level was followed by a period lasting about 20 Ma, in which sea-level decayed towards the 0 mark, which represents the present-day sea-level. We suggest that this gradual lowering of the sea-level can be attributed to downward deflection of the oceanic lithosphere, in response to the regional loading by the basalt extruded onto the sea-floor.
In a more recent publication, Rampino and his co-authors strengthened their original approach to cyclicity and the interrelation between various geological features, and introduced reference to impacts in a thoroughly forceful manner. Rampino et al. (1997) in their paper entitled 'A unified theory of impact crises and mass extinction: quantitive tests' state, 'the impact theory of mass extinctions makes several predictions that can be tested with available data'. They assert that the estimated number and the dynamics of Earth-
Figure 8.5 The 'kill curve' for Phanerozoic marine species plotted against energy of impacts named.
Figure 8.5 The 'kill curve' for Phanerozoic marine species plotted against energy of impacts named.
crossing asteroids and comets permit one to predict that impactors greater than a few kilometres in diameter will almost certainly collide with Earth every few tens of millions of years. Such large impacts will release >107 mt TNT equivalent of explosive energy, which is sufficient to produce global-scale disasters. They maintain that very large impact events, which release >108 mt TNT, produce additional severe effects, which include a complete loss of photosynthesis, an 'impact winter', as well as a global heat pulse brought about by wildfires caused by re-entering ejecta. They argue that the extinction record in the Phanerozoic, derived from paleontological studies, exhibits five major pulses and about 20 minor pulses, which these authors note are in excellent agreement with the predictions of about five major impact events of >108 mt yield and 25+/-5 impacts of 107-8 mt yields. (As we shall see later in this chapter, these figures are completely in agreement with our findings, that during the Phanerozoic we have evidence of 30 large impact events, some of which comprise multiple impacts.)
These authors present what they term the 'kill curve' (Figure 8.5). This is the result of a quantitative analysis which shows the relationship between the crater diameter in km (from which the energy of impact can be inferred) and the percentage extinction that can be expected. Two impacts (Puzech-Katunchi and Chicxulub) fall on the mean (solid line) curve, while the other known large impacts (Popigai and Manicouagan) fall on, or within, the dashed lines which represent the possible limits of error. Ward (1996) recently pointed out that this quantitative kill curve concept represents 'one of the most powerful to emerge from the entire extinction debate'.
Let us leave this aspect for the moment and see what can be gleaned by using data contained in the Atlas 3.3 system, to establish the incidence of abrupt track anomalies and hence, major impacts in the Phanerozoic.
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