Do we have a good idea how many impact events have affected life on Earth? New work on ocean sediments offers the chance to expand our knowledge, helping to flag the distinctive signature of an impact and even to tell us how large the incoming object was. We may find more historical impacts than have previously been identified, reminding us yet again that our habitable zone is an active and sometimes dangerous place to be.

True, the issues involved in mass extinctions are complicated, but major impacts clearly played a role in some, including the death of the dinosaurs. François Paquay and team estimate the impactor that struck 65 million years ago at the Cretaceous-Tertiary (K-T) boundary was between four and six kilometers in diameter. While other factors, including volcanism, can’t be ruled out, the meteorite certainly didn’t help matters.

Paquay (University of Hawaii at Manoa) analyzed samples of ocean sediments to study osmium levels therein. The element is useful because, as this news release explains, meteorites carry a distinct osmium isotope ratio that differs from that normally found in Earth’s oceans. Read properly, the osmium levels can provide a record of ancient impacts:

“The vaporization of meteorites carries a pulse of this rare element into the area where they landed,” says Rodey Batiza of the National Science Foundation (NSF)’s Division of Ocean Sciences, which funded the research along with NSF’s Division of Earth Sciences. “The osmium mixes throughout the ocean quickly. Records of these impact-induced changes in ocean chemistry are then preserved in deep-sea sediments.”

The critical issue is to verify how the impact changes the osmium levels, which is why Paquay has been focused on samples from the late Eocene, itself the time of the extinction event called the Grande Coupure, which may have been affected by impacts in Siberia and Chesapeake Bay. In any case, the period is known to have been marked by a number of impacts and thus offers the opportunity to observe the osmium signature related to these. Such studies led to the estimates of the K-T boundary event and may uncover the signs of previously unknown strikes.

The paper is Paquay et al., “Determining Chondritic Impactor Size from the Marine Osmium Isotope Record,” Science Vol. 320, No. 5873, pp. 214-218 (April 11, 2008). Abstract available.