Impact crises and mass extinctions: A working hypothesis

Based on evidence from astronomical observations, impact dynamics, and the geologic record, we explore a general hypothesis linking impacts of large asteroids and comets with mass extinctions of life. The probability of large-body impacts on the Earth derived from the flux of earth-crossing asteroids and comets and the estimated threshold impact size required to cause a global environmental disaster suggest that impacts of objects ≥ a few kilometers in diameter might be sufficient to explain the Phanerozoic record of extinction pulses. A number of extinction boundaries are known to be marked by severe environmental disturbances, including mass mortality and related extinctions (sometimes in steps), impoverished postextinction fauna and flora, and proliferation of stress-tolerant and opportunistic species, followed by gradual ecological recovery and radiation of new taxa. Abrupt negative shifts in δ¹³C in marine sedimentary rocks at extinction boundaries suggest major biomass losses followed by low-productivity " Strangelove" oceans, and fluctuations in δ¹⁸O may be interpreted as evidence of significant climatic oscillations. These biological, isotopic, and geochemical signatures seem to be consistent with the expected after effects of catastrophic impacts. Six of the extinction pulses may be associated with concurrent (in some cases multiple) impact markers (e.g., layers with high iridium, shocked minerals and/or microtektites, and large, dated impact craters). Elevated iridium levels at, or near, other extinction boundaries have characteristics suggesting a terrestrial origin, although they might be explained by collision of relatively low-Ir objects such as comets, and further work is warranted.