PROSPECTING FOR EVIDENCE OF IMPACT EVENTS IN THE MARINE SEDIMENT RECORD: HOW AND WHY

The osmium isotope composition (¹⁸⁷Os/¹⁸⁸Os) of pelagic sediments, coupled with platinum group element (PGE) concentration data, provides a powerful means of recognizing episodes of increased influx of extraterrestrial matter to the Earth. Marine records from the K-T boundary and the late Eocene demonstrate that large chondritic impact events greatly increase the inventory of dissolved Os and Ir in the global ocean, abruptly decreasing seawater ¹⁸⁷Os/¹⁸⁸Os and shifting the Os/Ir ratio of seawater to low non-steady state values. These distinctive shifts and subsequent recovery to a new steady state provide a means of distinguishing impact events from episodes of increased background flux of cosmic dust or intervals of extremely slow sedimentation. While it is unambiguous that large impact events impart a distinctive signal in the marine PGE record, the lower limit of impact detection is presently unknown. Calculations that assume more than half of the projectile-derived PGE inventory is soluble in seawater suggest that chondritic impacts as small as 2 km in diameter may be detectable in the marine Os isotope record.

Using Os isotopes and PGEs to recognize impact events can open new opportunities for documenting the environmental consequences of impact events by refining our knowledge of the timing of impact events beyond that achievable by radiometric dating. The episode of late Eocene impact events provides a clear example of how correlation of an impact event into the global magnetobiostratigraphy is an essential first step toward gauging the response of the Earth system to abrupt perturbations caused by impact events. Although it has been long recognized that the late Eocene impacts did not cause significant extinction, recently published work by other groups suggest that these impacts coincided with disruption of the marine carbon cycle. The stage is now set for determining if this carbon cycle disruption extends to the terrestrial realm and seeking evidence of the effects the late Eocene impacts had on terrestrial environments distributed throughout the globe. We are currently seeking other impact induced Os isotope excursions in order to expand the number of impact events for which global effects can be recognized.