An extensive, but incompletely preserved, record of impact craters on the Earth demonstrates that large projectiles from space have repeatedly hit Earth?s surface. Although the Cretaceous-Tertiary mass extinction some 66 million years ago coincides precisely with the largest known impact event of the last 500 million years, the environmental consequences of numerous other large impact events remain poorly understood and documented. In order to study the response of the Earth system to these large, abrupt perturbations, it is essential to be able to reliably recognize an impact horizon globally throughout the sediment record. Currently elevated iridium (Ir) concentrations are most commonly used to try to recognize impact horizons far removed from the site of impact, but the results are often ambiguous. The research supported by this award will test the usefulness of combining measurements of Osmium (Os) isotope ratios (187Os/188Os) with Os and Ir concentrations to reliably identify impact horizons, and also to estimate projectile size. This will be accomplished by performing these analyses in well studied marine sediment sequences that are known to have accumulated during the time that large impact craters formed. Some of the sites to be studied are relatively close to the impact site while others are located thousands of kilometers away. PI's previous work using Os isotopes to recognize impact events in deep sea sediments composed mainly of calcium carbonate microfossils have yielded promising results. In this new study he will investigate if compositionally different types of marine sediments can also reliably record the chemical signatures of large impact events. Completion of these research objectives will show if the marine Os isotope record represents a new and unparalleled record of impact events through Earth history, or simply a modest augmentation of the often ambiguous record of impacts obtainable through the study of Ir concentration data alone. If the former is true this work will make lasting contributions to event stratigraphy and global correlations of the sediment record, as well as to understanding abrupt perturbations of Earth's climate system by large impact events.
The aim of this proposal was to further develop the marine osmium (Os) isotope record as tool for recognizing large extraterrestrial impact events in the geologic record. Although we did not accomplish all of our goals, we did make important contributions to the study of impact events, and study of the geochemistry of sedimentary rocks in general. One important contribution is linked to ongoing efforts to better understand the consequences of the well-known mass extinction at the Cretaceous-Paleogene boundary approximately 66 million year ago. Determining how quickly new species evolve and the ecosystems recover following a mass extinction is very difficult, but it is also an important part of understanding how resilient the Earth system is to major catastrophes. We made Os isotope measurements and developed a simple model that provides a new approach to estimating recovery time. Comparing our time estimates to independent approaches for estimating recovery time yields similar results, indicating geologists can be confident that the recovery process takes at least tens of thousands of years in the deep ocean. A second important contribution is related to an abrupt change in Earth’s climate roughly 11,000 years ago called the Younger Dryas event. Some research’s argue that this rapid cooling event in the consequence of a large extraterrestrial impact event. We published results showing that previous reports of elevated iridium in sedimentary deposits of this age could not be reproduced. Os concentrations were not elevated either, and measured 187Os/188Os carried the fingerprint of typical rocks from Earth’s surface and not extraterrestrial material. These results are important because they require that if a large impact occurred it must have been an extraterrestrial body that is different than the most common types of meteorites. Climate scientists are eager to learn if abrupt climate change events like the Younger Dryas can develop in response to changing conditions on the Earth, or if they require large external disruptions like an impact event to occur. This is one reason our failure to substantiate evidence of a large impact event coincident with Younger Dryas cooling is of interest to many scientists. Our final contribution was a methodological development for the rapid measurement of rhenium concentrations, osmium concentrations and Os isotope ratios in a wide variety of marine sediments. This work has received interest from workers in the oil and gas industry who are working to more cost effectively recover oil and natural gas from organic-rich shales. They are interested in our method because it may be a valuable tool for correlating productive intervals in different cores. Prior to our work it was too expensive to pursue this possibility, but the faster and less costly methods we have developed warrant further investigation. With regard to educational results, this award partially supported the research and education of 3 graduate students, 2 at the University of Hawaii and a visiting student from Penn State University. One of these students has completed a PhD degree, one a Masters degree and the third is currently working toward completion of a Masters degree. This award also fostered international collaboration with colleagues in Japan, Sweden and Belgium. In addition this project supported or partially supported eight papers published in peer-reviewed, scholarly journals and 2 additional manuscripts that have been submitted and are currently in review.
