Cosmic Dust in Seafloor Sediments

Geochemical investigations using the element Iridium have demonstrated that the major species extinction horizon that marks the end of the Cretaceous Era (65 million years ago) was caused by the catastrophic impact of a large extraterrestrial object with the Earth. This event demonstrates that the Earth's biological and climatic systems are very sensitive to the accretion of material from space. While Iridium provides a tool for recognizing large, episodic impact events, it is not very sensitive to the large and temporally continuous accretion of cosmic dust (photomicrograph, left, showing 10 micron grain) submillimeter grains which rain down at a rate of about 40 million kg per year.

We have been working to determine the accretion rate of cosmic dust back through time, by analyzing the rare isotope 3He in deep sea sediments. This isotope is extremely abundant in extraterrestrial matter, and in most seafloor sediments more than 95% of the 3He derives from cosmic dust48. My results reveal striking variations in the cosmic dust accretion rate with time, which, intriguingly, are temporally correlated with both large impact events15 and global glacial cycles17. I am presently working to understand whether these correlations indicate causality, or are simply coincidental. Confirmation of a causal relationship would provide critical insights to the behavior and sensitivity of the Earth system to extraterrestrial events. My most recent work seems to suggest the occurrence of a comet shower - a period of strongly enhanced cometary activity, possibly caused by gravitational perturbations associated with passage of a star close to our solar system - at 36 million years ago34. The K/T boundary impact was NOT associated with such a shower, nor is there any indication of comet shower periodicity, at least in the period 30 to 74 Ma49.

A more recent application of this technique is to use extraterrestrial 3He abundances as an indicator of sedimentation rate (high sedimentation rates yield low 3He concentrations for a given extraterrestrial flux, and vice versa), particularly for assessing the pace of rapid climate change events in the distant geologic past. Using this technique we can constrain the duration of the K/T boundary clay to just 20 kyrs49! Similar work provided new constraints on the duration and temporal progression of the Paleocene-Eocene Thermal Maximum, a very large and rapid climate excusion that occurred 55 million years ago70.

Several claims of extraterrestrial impacts at the Permian-Triassic Boundary have been made, some based on detection of extremely high 3He concentrations in boundary sediments. Sometimes the 3He is thought to be encapsulated in fullerenes. Although we have repeatedly tried 56,82 we have been unable to confirm high 3He levels in any Permian-Triassic boundary sediments, casting doubt on the earlier reports.

In contrast there is abundant evidence from 3He and from direct observations of the asteroid belt for a massive asteroid collision 8.2 Million years ago. This collision produced the Veritas asteroid family and greatly enhanced the flux of extraterrestrial dust to Earth for a several million year period89.

Participants in my group include:

Sujoy Mukhopadhyay now at Harvard