The Incomplete Impact Record and Implications for Ice Core Studies

The impact risk is extremely uncertain for objects of order 0.1-1 km diameter, with kinetic energies in the range 100 to 1 million Mt (megaton TNT ~ 4×1015 J) and recurrence times estimated in thousands to many tens of thousands of years. Millennial timescales are especially interesting, since the character of explosions (e.g. impacts, large volcanic eruptions) that only occur every 103 to 104 years lies just beyond the reckoning of modern cultural history. The impact rate predicted for the Earth based on observing nearby objects is much higher than the endemic rate estimated by counting known craters on Earth's surface. We have examined the latest account of confirmed craters from the Earth Impact Database (http://www.unb.ca/passc/ImpactDatabase/) over the last 100 Ma. The cratering record contains a large gap between 35 and 5 Ma, during which the apparent impact rate drops by an order of magnitude. The gap occurs during a period of substantial climate change, notably the initiation of large scale permanent glaciers, based on climate proxies from deep-sea sediment cores. A likely partial explanation is that climate change eroded or precluded crater formation in the recent geologic past. Taken together with constraints from inner solar system cratering and observations of near earth objects, the apparent gap in crater formation suggests that the terrestrial impact record is grossly incomplete over timescales much shorter than 100 Ma. If the true impact rate is more commensurate with the higher rates inferred from the local planetary environment, then some of the explosive fallout layers now observed in ice cores may actually be the result of recent impacts rather than volcanic eruptions. Like very large eruptions, impact ejecta are likely to be widely distributed, since impactors disrupt all levels of the atmosphere and generate ballistic debris and vapor plumes that can rise above the stratosphere. Polar ice core records of the last ~50-100 ka have become sufficiently extensive and synchronized to suggest an emerging pattern between explosive volcanic layers and abrupt climate change, although atmospheric focusing and preservation of deposits complicate analysis. By comparing ice core ash and acid signals of explosive volcanic fallout in Antarctica and Greenland, we have identified a number of common horizons which likely resulted from "eruptions" that dispersed material world-wide. These events appear to have preferentially occurred during onsets or intensifications of cooling phases, contributing to long-standing debates over possible feedbacks between climatic and geologic phenomena. The correlation may be evidence that abrupt climate change and associated changes in sea level and ice sheets can trigger volcanism. An intriguing and conspicuous alternative possibility is that catastrophic explosions force long term cooling. A relatively high rate for small impact events (objects hundreds of meters in size) could imply that impacts (instead of volcanism) are a causal factor in some of the climate variations observed during the last million years. Our method of optically probing outward from the boreholes created by ice coring missions is capable of detecting particulate layers which are invisible or largely missing in the cores, identifying candidate horizons for chemical analyses. Deliberate and discriminative analysis of the "ash" layers in ice cores could shed light on the recent impact rate. This research was supported by grant NSF ANT-0440609.