Six years have passed since L.W. Alvarez and his colleagues presented evidence suggesting that a 10-km asteroid had struck the earth 65 million years ago and created a global dust cloud that suppressed photosynthesis and led to the extinctions associated with the Cretaceous/Tertiary (K/T) boundary [Alvarez et al., 1980]. Their evidence was anomalous amounts of iridium and other siderophile elements at K/T sections in Italy, Denmark, and New Zealand. Since then, the single asteroid impact hypothesis has evolved into a hypothesis of periodic comet showers, each shower consisting of several cometary impacts over a 1–2-million-yr period, to explain not only the K/T but the Eocene/Oligocene (E/O) as well as other extinction events recorded throughout the Phanerozoic [Davis et al., 1984; Hut et al., 1985].
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