2003 Annual Science Report
University of Washington Reporting | JUL 2002 – JUN 2003
Causes of Mass Extinction: Isotopic and Paleontological Constraints
During the grant period data gathering trips were conducted in Italy, (Triassic-Jurassic boundary site), with co-I Ken Farely; to the Permian /Triassic boundary in Alberta with National Research Council (NRC) post Doc Geoff Garrison, and to the New York Canyon region of Nevada, with co-I David Kring. Paleontological, isotopic, and chemical studies of all three sites are either underway in our labs or now completed. Geochemical analyses of last year’s sampling at the Kennecott Point section (Triassic/Jurassic) have now been completed, while geochem from last year’s trip to the Karoo of South Africa is now completed and is ready for publication. At the University of Arizona we have been preparing and analyzing samples collected from the Queen Charlotte Islands for petrologic studies using thin-sections. Mineral separations are also underway, so that we can examine the samples for evidence of impact-induced shock metamorphism.
The Triassic-Jurassic (T/J) boundary is a particularly interesting target for our work because an impact event, and possibly multiple impact events, may be associated with this major extinction boundary. The occurrence of multiple impact events over a short period is suggestive of a comet shower. In previous work (our Year 1) we analyzed T/J samples from the Queen Charlotte Islands, and found no evidence for extraterrestrial 3He. However we are concerned that the Queen Charlotte samples were sufficiently heated by burial to release the extraterrestrial 3He from its carrier minerals, so we have analyzed samples from several other localities. In the summer of 2002 we collected about 100 limestone samples for extraterrestrial 3He analysis from two important T/J sections in northwestern Italy. Our analyses of these rocks reveal only a very small extraterrestrial 3He signal. While there appears to be elevated extraterrestrial 3He levels in two of the shocked-quartz bearing shales, the effect is small and may result solely from changes in accumulation rate. These data provide no new evidence for a comet shower. Taken together these observations provide no evidence at all for an extraterrestrial impact at the T/J boundary.
Perhaps the strongest recent evidence for a role of impacts at major extinction boundaries is the work of Becker et al. reporting extraterrestrial fullerenes, with encapsulated extraterrestrial noble gases, at both the Cretaceous-Tertiary boundary and Permian-Triassic (P/T) boundary. The latter is especially important because it is among the few clues to the cause of the biggest extinction event in Earth history. We previously reported our inability to reproduce the Permian-Triassic results. In the last year we developed a new technique by which to evaluate the key question of fullerene He retentivity. He bearing fullerenes were made either by synthesizing them in a He atmosphere, or by subjecting fullerenes to high He pressures at elevated temperatures. We took He-bearing fullerenes and measured the He release as a function of temperature from about 50 to 250°C. Our hope was to obtain a linear Arrhenius relation from which we could estimate the release rate under natural conditions. We found that the release pattern does not obey a linear Arrhenius relation, making extrapolation impossible. Instead it appears that a major change in behavior occurs as the sample is heated. Based on reports in the literature we believe this pattern arises from traces of organic solvents used in the purification of the fullerenes, which degrade the fullerenes at elevated temperatures.
PROJECT INVESTIGATORS:Peter Ward
PROJECT MEMBERS:Roger Buick
RELATED OBJECTIVES:Objective 4.2
Foundations of complex life
Effects of extraterrestrial events upon the biosphere
Environmental changes and the cycling of elements by the biota, communities, and ecosystems