Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 250

period but the validity of these findings has often been disputed. Possible impact
craters are also either incompletely documented or do not correlate in age. Expla-
nations for the event range from oceanic anoxia, oceanic CO 2 ,tooverturn and
eruption of flood basalts from the Siberian Traps. Retallack et al. (1998)found
similarities in the isotopic composition of the boundary beds (6--15 cm thick
claystone breccia) at locations in both Antarctica and Australia. These beds are
interpreted as redeposited soils as they do not contain spherules or other indi-
cators of impact ejecta. However, the claystone breccias do contain rare grains of
shocked quartz and a small iridium anomaly was identified. Unfortunately, the
Retallack et al. (1998)study did not shed any further light on the mass extinction
event at the P--T boundary but a 'hint' of an impact was revealed.
Conclusion
Asteroid and significant meteorite impacts with Earth are uncommon
events and rarely taken seriously in terms of any risk assessments. The prehistoric
record shows that truly catastrophic events have occurred in the past and no
doubt will do so in the future. There is little doubt that this hazard needs to
be taken more seriously but until such an event occurs, even if it only causes
relatively minor damage or a few deaths, governments and public authorities
are unlikely to ever give this threat serious consideration. The enormity of the
potential consequences and their very low frequency of occurrence are no doubt
instrumental in the propagation of this attitude. However, the unquestionable
origin of much of the evidence such as shock structures in combination with
element anomalies and physically imposing features, such as craters, show us
that mitigation against this hazard will require very sophisticated technology
and be of great expense.