2004 Annual Science Report
University of California, Los Angeles Reporting | JUL 2003 – JUN 2004
Extraterrestrial Impact History on Earth
Field studies in the Barberton greenstone belt, South Africa , were conducted in June and July 2003 by Prof. Donald R. Lowe, Stanford University , and Prof. Gary Byerly, Louisiana State University , on 1) the Archean impact record, especially very large impacts; 2) Archean komatiitic volcanism; and 3) surface processes on the Archean Earth. In addition, Lowe and Byerly led a field conference for 29 invited international scientists June 23- July 2, 2003 , in the Barberton belt. This conference was sponsored by the NAI and partially supported by the UCLA NAI program. Among the key issues addressed during the conference were the relative roles of biological and abiotic (hydrothermal) processes in forming carbonaceous matter and siliceous sediments in the Barberton and similar early greenstone-belt sequences. Byerly has concentrated on: 1) refinement of field mapping, stratigraphy, and geochronology of the Barberton greenstone belt; and 2) examination of the Barberton ironstone pods, including field relations, mineralogy, and geochemistry. We have isolated zircons from several critical horizons, one a possible new impact layer, and one that will provide important constraints on the rates of deposition of black cherts. These samples should be dated this year using the Stanford Sensitive, High-Resolution Ion Microprobe (SHRIMP) facility. During the academic year, progress was made in writing and publishing the results of our environmental studies, and Mrs. Alexandra Krull Davatzes, a Stanford PhD student, began writing chapters in her thesis dealing with Archean impacts and impact processes. She has submitted one paper to Geology dealing with the condensation mechanics of rock vapor plumes formed by large meteorite impacts. Louisiana State University (LSU) PhD student Keena Kareem, is nearing completion of her work on these komatiites — several highlights are listed below. Two new LSU students are beginning work, including one that will focus on near-surface Archean alteration, especially carbonation of komatiites.
Frank T. Kyte, UCLA, has carried out analytical work on the geochemistry of sediments from several known impact and extinction horizons ranging in age from late Pliocene (2.4 Ma) through early Archean (3.5 Ga). This has included work on sediments from mass extinction events at the Permian-Triassic, Triassic-Jurassic, and Cretaceous-Tertiary boundaries. An important project concluding this year has been characterization of impact deposits from one of the late Eocene impact events. Previous work on the late Eocene provided strong evidence of increased flux of interplanetary dust during the late Eocene, which might be caused by a comet shower from the Oort Cloud at the outer fringes of the solar system. Our latest work shows that the Cr-isotopic signature from a late Eocene impactor is consistent with a source from a main-belt asteroid compositionally similar to ordinary chondrites. We suggest that the increased dust flux might be related to numerous asteroid collisions following a major breakup event in the asteroid belt.